Race in NettyRpcConnection may cause call stuck in BufferCallBeforeInitHandler forever

Found this when implementing the POC of HDFS-9924.

The problem is that channel.writeAndFlush will find the first outbound handler before scheduling the actual write task into event loop, so it is possible that when we actually execute the write task, the first outbound handler has already been changed but the task still refers to the old handler which has already been removed.