Fix releasing of resources in AioEventLoopGroup, as it missed to shutdown the AsynchronousChannelGroup before. See #659

transport/src/main/java/io/netty/channel/socket/aio/AioEventLoopGroup.java

@@ -71,6 +71,13 @@ public class AioEventLoopGroup extends MultithreadEventLoopGroup {
         }
     }
 
+    @Override
+    public void shutdown() {
+        super.shutdown();
+        // Also shutdown the underlying AsynchrounsChannelGroup
+        group.shutdown();
+    }
+
     @Override
     protected EventExecutor newChild(
             ThreadFactory threadFactory, ChannelTaskScheduler scheduler, Object... args) throws Exception {