Make Epoll ChannelMetadata more consistent with NIO

Motivation:
In 4.0 AbstractNioByteChannel has a default of 16 max messages per read. However in 4.1 that constraint was applied at the NioSocketChannel which is not equivalent. In 4.1 AbstractEpollStreamChannel also did not have the default of 16 max messages per read applied.

Modifications:
- Make Nio consistent with 4.0
- Make Epoll consistent with Nio

Result:
Nio and Epoll both have consistent ChannelMetadata and are consistent with 4.0.

transport-native-epoll/src/main/java/io/netty/channel/epoll/AbstractEpollStreamChannel.java

@@ -22,6 +22,7 @@ import io.netty.channel.Channel;
 import io.netty.channel.ChannelConfig;
 import io.netty.channel.ChannelFuture;
 import io.netty.channel.ChannelFutureListener;
+import io.netty.channel.ChannelMetadata;
 import io.netty.channel.ChannelOutboundBuffer;
 import io.netty.channel.ChannelPipeline;
 import io.netty.channel.ChannelPromise;
@@ -51,7 +52,7 @@ import static io.netty.channel.unix.FileDescriptor.pipe;
 import static io.netty.util.internal.ObjectUtil.checkNotNull;
 
 public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel implements DuplexChannel {
-
+    private static final ChannelMetadata METADATA = new ChannelMetadata(false, 16);
     private static final String EXPECTED_TYPES =
             " (expected: " + StringUtil.simpleClassName(ByteBuf.class) + ", " +
                     StringUtil.simpleClassName(DefaultFileRegion.class) + ')';
@@ -130,6 +131,11 @@ public abstract class AbstractEpollStreamChannel extends AbstractEpollChannel im
         return new EpollStreamUnsafe();
     }
 
+    @Override
+    public ChannelMetadata metadata() {
+        return METADATA;
+    }
+
     /**
      * Splice from this {@link AbstractEpollStreamChannel} to another {@link AbstractEpollStreamChannel}.
      * The {@code len} is the number of bytes to splice. If using {@link Integer#MAX_VALUE} it will

transport-udt/src/main/java/io/netty/channel/udt/nio/NioUdtByteConnectorChannel.java

@@ -44,8 +44,6 @@ public class NioUdtByteConnectorChannel extends AbstractNioByteChannel implement
     private static final InternalLogger logger =
             InternalLoggerFactory.getInstance(NioUdtByteConnectorChannel.class);
 
-    private static final ChannelMetadata METADATA = new ChannelMetadata(false, 16);
-
     private final UdtChannelConfig config;
 
     public NioUdtByteConnectorChannel() {
@@ -175,11 +173,6 @@ public class NioUdtByteConnectorChannel extends AbstractNioByteChannel implement
         return javaChannel().socket().getLocalSocketAddress();
     }
 
-    @Override
-    public ChannelMetadata metadata() {
-        return METADATA;
-    }
-
     @Override
     protected SocketAddress remoteAddress0() {
         return javaChannel().socket().getRemoteSocketAddress();

transport/src/main/java/io/netty/channel/nio/AbstractNioByteChannel.java

@@ -20,6 +20,7 @@ import io.netty.buffer.ByteBufAllocator;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelConfig;
 import io.netty.channel.ChannelFuture;
+import io.netty.channel.ChannelMetadata;
 import io.netty.channel.ChannelOption;
 import io.netty.channel.ChannelOutboundBuffer;
 import io.netty.channel.ChannelPipeline;
@@ -36,7 +37,7 @@ import java.nio.channels.SelectionKey;
  * {@link AbstractNioChannel} base class for {@link Channel}s that operate on bytes.
  */
 public abstract class AbstractNioByteChannel extends AbstractNioChannel {
-
+    private static final ChannelMetadata METADATA = new ChannelMetadata(false, 16);
     private static final String EXPECTED_TYPES =
             " (expected: " + StringUtil.simpleClassName(ByteBuf.class) + ", " +
             StringUtil.simpleClassName(FileRegion.class) + ')';
@@ -63,6 +64,11 @@ public abstract class AbstractNioByteChannel extends AbstractNioChannel {
         return new NioByteUnsafe();
     }
 
+    @Override
+    public ChannelMetadata metadata() {
+        return METADATA;
+    }
+
     protected class NioByteUnsafe extends AbstractNioUnsafe {
 
         private void closeOnRead(ChannelPipeline pipeline) {

transport/src/main/java/io/netty/channel/socket/nio/NioSocketChannel.java

@@ -48,7 +48,6 @@ import java.util.concurrent.Executor;
  */
 public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(NioSocketChannel.class);
-    private static final ChannelMetadata METADATA = new ChannelMetadata(false, 16);
     private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
 
     private static SocketChannel newSocket(SelectorProvider provider) {
@@ -104,11 +103,6 @@ public class NioSocketChannel extends AbstractNioByteChannel implements io.netty
         return (ServerSocketChannel) super.parent();
     }
 
-    @Override
-    public ChannelMetadata metadata() {
-        return METADATA;
-    }
-
     @Override
     public SocketChannelConfig config() {
         return config;