Purge FutureReturnValueIgnored warnings from error prone

We used to introduce a FutureUtils to purge the FutureReturnValueIgnored warnings in the async client implementation, to make sure we catch the exception from the callback.

For netty, we could apply the same trick. And there is another problem for netty that channel.write and some other methods will also return a ChannelFuture but usually we just do nothing on it, this will generated a FutureReturnValueIgnored warning too. We could add a logging listener to log the errors if any.