Blocking on Communication

[xiang90]

I read through the communication layer.

I think the approach is fine.
Register a RPC paired with channel. Switch on the msg type when receive from TCP , decoding and send to channel.
The upper layer can just use a select on all the receiving channels.
That is very similar to what I have done in raft.

I think one thing we can improve here is that we can try a non-blocking sendMsg when doing broadcast. This can reduce the overall latency if the underlay tcp buffer is full.
But there might be some reason you did a blocking one, so I create this issue.

Also I remember @imoraru talked about multi-threading improvement. But I forgot the details...