[#2797] Let EventExecutorGroup extend AutoCloseable

Motivation:

To make it more easy to shutdown an EventExecutorGroup / EventLoopGroup we should let both of them extend AutoCloseable.

Modifications:

Let EventExecutorGroup extend AutoCloseable and impement it.

Result:

Easier shutdown of EventExecutorGroup and EventLoopGroup

common/src/main/java/io/netty/util/concurrent/AbstractEventExecutor.java

@@ -154,4 +154,9 @@ public abstract class AbstractEventExecutor extends AbstractExecutorService impl
     public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) {
         throw new UnsupportedOperationException();
     }
+
+    @Override
+    public void close() throws Exception {
+        shutdownGracefully().syncUninterruptibly();
+    }
 }

common/src/main/java/io/netty/util/concurrent/AbstractEventExecutorGroup.java

@@ -114,4 +114,9 @@ public abstract class AbstractEventExecutorGroup implements EventExecutorGroup {
     public void execute(Runnable command) {
         next().execute(command);
     }
+
+    @Override
+    public void close() throws Exception {
+        shutdownGracefully().syncUninterruptibly();
+    }
 }

common/src/main/java/io/netty/util/concurrent/EventExecutorGroup.java

@@ -27,7 +27,7 @@ import java.util.concurrent.TimeUnit;
  * to shut them down in a global fashion.
  *
  */
-public interface EventExecutorGroup extends ScheduledExecutorService {
+public interface EventExecutorGroup extends ScheduledExecutorService, AutoCloseable {
 
     /**
      * Returns {@code true} if and only if all {@link EventExecutor}s managed by this {@link EventExecutorGroup}

transport/src/main/java/io/netty/channel/PausableChannelEventExecutor.java

@@ -326,6 +326,11 @@ abstract class PausableChannelEventExecutor implements PausableEventExecutor, Ch
         unwrap().execute(command);
     }
 
+    @Override
+    public void close() throws Exception {
+        unwrap().close();
+    }
+
     private static final class ChannelCallableEventExecutor<V> implements CallableEventExecutorAdapter<V> {
 
         final Channel channel;