Reduce Memory Usage of SingleThreadEventLoopTest

Motivation:

The SingleThreadEventLoopTest allocated up to half a gigabyte of memory per run,
causing up to 50 GC runs in ~20 seconds.

Modification:

Charlie Hunt identified TLAB allocations to be the root cause of this excessive
memory usage [1]. By reusing the Executor in every test we can reduce the
memory usage by >50%.

Result:

Lower Memory Usage and fewer Garbage Collector runs.
Helps to resolve GitHub issue #2841.

[1] https://twitter.com/charlesjhunt/status/505351389317722112

transport/src/test/java/io/netty/channel/SingleThreadEventLoopTest.java

@@ -19,10 +19,13 @@ import ch.qos.logback.classic.Logger;
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.core.Appender;
 import io.netty.channel.local.LocalChannel;
+import io.netty.util.concurrent.DefaultExecutorFactory;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.concurrent.PausableEventExecutor;
 import org.junit.After;
+import org.junit.AfterClass;
 import org.junit.Before;
+import org.junit.BeforeClass;
 import org.junit.Test;
 import org.slf4j.LoggerFactory;
 
@@ -32,7 +35,7 @@ import java.util.List;
 import java.util.Queue;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Executors;
+import java.util.concurrent.Executor;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ScheduledFuture;
@@ -52,9 +55,16 @@ public class SingleThreadEventLoopTest {
         public void run() { }
     };
 
+    private static Executor executor;
+
     private SingleThreadEventLoopA loopA;
     private SingleThreadEventLoopB loopB;
 
+    @BeforeClass
+    public static void newExecutor() {
+        executor = new DefaultExecutorFactory("SingleThreadEventLoopTest").newExecutor(2);
+    }
+
     @Before
     public void newEventLoop() {
         loopA = new SingleThreadEventLoopA();
@@ -689,7 +699,7 @@ public class SingleThreadEventLoopTest {
         final AtomicInteger cleanedUp = new AtomicInteger();
 
         SingleThreadEventLoopA() {
-            super(null, Executors.newSingleThreadExecutor(), true);
+            super(null, executor, true);
         }
 
         @Override
@@ -719,7 +729,7 @@ public class SingleThreadEventLoopTest {
         private volatile boolean interrupted;
 
         SingleThreadEventLoopB() {
-            super(null, Executors.newSingleThreadExecutor(), false);
+            super(null, executor, false);
         }
 
         @Override