Add an Epoll benchmark

Motivation:
Optimizing the Epoll channel needs an objective measure of how fast
it is.

Modification:
Add a simple, closed loop,  ping-pong benchmark.

Result:
Benchmark can be used to measure #7816

Initial numbers:

```
Result "io.netty.microbench.channel.epoll.EpollSocketChannelBenchmark.pingPong":
  22614.403 ±(99.9%) 797.263 ops/s [Average]
  (min, avg, max) = (21093.160, 22614.403, 24977.387), stdev = 918.130
  CI (99.9%): [21817.140, 23411.666] (assumes normal distribution)

Benchmark                              Mode  Cnt      Score     Error  Units
EpollSocketChannelBenchmark.pingPong  thrpt   20  22614.403 ± 797.263  ops/s
```

microbench/pom.xml

@@ -32,6 +32,9 @@
     <!-- Skip tests by default; run only if -DskipTests=false is specified -->
     <skipTests>true</skipTests>
     <jmh.version>1.19</jmh.version>
+    <!-- This only be set when run on linux as on other platforms we just want to include the jar without native
+         code -->
+    <epoll.classifier/>
   </properties>
 
   <profiles>
@@ -42,6 +45,9 @@
           <family>linux</family>
         </os>
       </activation>
+      <properties>
+        <epoll.classifier>${jni.classifier}</epoll.classifier>
+      </properties>
       <build>
         <plugins>
           <plugin>
@@ -78,6 +84,12 @@
       <artifactId>netty-codec-redis</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>${project.groupId}</groupId>
+      <artifactId>netty-transport-native-epoll</artifactId>
+      <version>${project.version}</version>
+      <classifier>${epoll.classifier}</classifier>
+    </dependency>
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>

microbench/src/main/java/io/netty/microbench/channel/epoll/EpollSocketChannelBenchmark.java

@@ -0,0 +1,139 @@
+/*
+ * Copyright 2018 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.microbench.channel.epoll;
+
+import io.netty.bootstrap.Bootstrap;
+import io.netty.bootstrap.ServerBootstrap;
+import io.netty.buffer.ByteBuf;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelDuplexHandler;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.ChannelInitializer;
+import io.netty.channel.ChannelPromise;
+import io.netty.channel.epoll.EpollEventLoopGroup;
+import io.netty.channel.epoll.EpollServerSocketChannel;
+import io.netty.channel.epoll.EpollSocketChannel;
+import io.netty.microbench.util.AbstractMicrobenchmark;
+import io.netty.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.TearDown;
+
+public class EpollSocketChannelBenchmark extends AbstractMicrobenchmark {
+
+    private EpollEventLoopGroup group;
+    private Channel serverChan;
+    private Channel chan;
+    private ByteBuf abyte;
+    private ScheduledFuture<?> future;
+
+    @Setup
+    public void setup() throws Exception {
+        group = new EpollEventLoopGroup(1);
+
+        // add an arbitrary timeout to make the timer reschedule
+        future = group.schedule(new Runnable() {
+            @Override
+            public void run() {
+                throw new AssertionError();
+            }
+        }, 5, TimeUnit.MINUTES);
+        serverChan = new ServerBootstrap()
+            .channel(EpollServerSocketChannel.class)
+            .group(group)
+            .childHandler(new ChannelInitializer<Channel>() {
+                @Override
+                protected void initChannel(Channel ch) {
+                    ch.pipeline().addLast(new ChannelDuplexHandler() {
+                        @Override
+                        public void channelRead(ChannelHandlerContext ctx, Object msg) {
+                            if (msg instanceof ByteBuf) {
+                                ctx.writeAndFlush(msg, ctx.voidPromise());
+                            } else {
+                                throw new AssertionError();
+                            }
+                        }
+                    });
+                }
+            })
+            .bind(0)
+            .sync()
+            .channel();
+    chan = new Bootstrap()
+        .channel(EpollSocketChannel.class)
+        .handler(new ChannelInitializer<Channel>() {
+            @Override
+            protected void initChannel(Channel ch) {
+                ch.pipeline().addLast(new ChannelDuplexHandler() {
+
+                private ChannelPromise lastWritePromise;
+
+                    @Override
+                    public void channelRead(ChannelHandlerContext ctx, Object msg) {
+                        if (msg instanceof ByteBuf) {
+
+                            ByteBuf buf = (ByteBuf) msg;
+                            try {
+                                if (buf.readableBytes() == 1) {
+                                    lastWritePromise.trySuccess();
+                                    lastWritePromise = null;
+                                } else {
+                                    throw new AssertionError();
+                                }
+                            } finally {
+                                buf.release();
+                            }
+                        } else {
+                            throw new AssertionError();
+                        }
+                    }
+
+                    @Override
+                    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise)
+                            throws Exception {
+                        if (lastWritePromise != null) {
+                            throw new IllegalStateException();
+                        }
+                        lastWritePromise = promise;
+                        super.write(ctx, msg, ctx.voidPromise());
+                    }
+                });
+            }
+        })
+        .group(group)
+        .connect(serverChan.localAddress())
+        .sync()
+        .channel();
+
+        abyte = chan.alloc().directBuffer(1);
+        abyte.writeByte('a');
+    }
+
+    @TearDown
+    public void tearDown() throws Exception {
+        chan.close().sync();
+        serverChan.close().sync();
+        future.cancel(true);
+        group.shutdownGracefully(0, 0, TimeUnit.SECONDS).sync();
+        abyte.release();
+    }
+
+    @Benchmark
+    public Object pingPong() throws Exception {
+        return chan.pipeline().writeAndFlush(abyte.retainedSlice()).sync();
+    }
+}

microbench/src/main/java/io/netty/microbench/channel/epoll/package-info.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2018 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+/**
+ * Benchmarks for {@link io.netty.microbench.channel.epoll}.
+ */
+package io.netty.microbench.channel.epoll;