Correct the return type of MultithreadEventLoopGroup.newChild()

Motivation:

MultithreadEventLoopGroup.newChild() does not override MultithreadEventExecutorGroup.newChild() which returns EventExecutor.  MultithreadEventLoopGroup.newChild() should never return an EventExecutor, so this is incorrect.

Modifications:

Override MultithreadEventLoopGroup.newChild() so that it returns EventLoop

Result:

Correct API

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

@@ -67,6 +67,9 @@ public EventLoop next() {
         return (EventLoop) super.next();
     }
 
+    @Override
+    protected abstract EventLoop newChild(Executor executor, Object... args) throws Exception;
+
     @Override
     public ChannelFuture register(Channel channel) {
         return next().register(channel);