RFC: Add streaming APIs (RpcClient, RpcServer)

[fengjiachun]

Add streaming APIs (RpcClient, RpcServer), implement it based on gRPC.

This is a subtask that belongs to the first step of #672
In this subtask, we need a streaming impl, for the leader (client) to continuously send raft logs to followers.

Main goals:

• Add streaming API to RpcClient and RpcServer
• Implement streaming based on gRPC (We are not going to use bolt to implement)

[sollhui]

@fengjiachun I want to have a try，can you assign me？And I have said in #672 before

[sollhui]

@fengjiachun I hava some idea to discuss:
when using streaming api to send ,Buffer overflow will be generated when processing large-scale stream data,reactive stream provides a set of standard asynchronous stream processing api to suppuort backpressure and it can ensures the stability of nodes,so whether use Reactive Stream to make encapsulation of grpc would be a good idea?

[fengjiachun]

@HHoflittlefish777
In the case of jraft replicating raft log, I think backpressure may not be needed, so I'm not sure if it is a good idea. Can we implement basic grpc streaming first and then think about reactive streaming?

[fengjiachun]

Slack

[sollhui]

@HHoflittlefish777 In the case of jraft replicating raft log, I think backpressure may not be needed, so I'm not sure if it is a good idea. Can we implement basic grpc streaming first and then think about reactive streaming?

OK,I do it first