Support follower linearizable read

The RaftPHD Thesis section 6.4 proposes a approach for follower linearizable read. Despite the fact that leader has to make all write decesions, followers could help offload the processing of read-only queries. This would improve system's read throughput, and it would also dovert load away from leader, allowing the leader to process more read-write requests.

According to experiments conducted by PingCAP, enabling follower read double the read throuput of the entire system. see source.

In this proposal I hope the bring the Follower Linerizable Read feture to the Apache Ratis Community.