Support fully concurrent read for raft

[windshg]

Supporting Fully concurrent read through all replicas for one raft group can improve
reading performance significantly.

[dangleptr]

Yes, we could support reading on follower

[bjzhaoqing]

For example in dgraph, follower nodes can serve read requests. In Ludicrous mode, even if the cluster is down, as long as there is a meta node, a graphd node and a storage node, the cluster can still provide read services. Of course, writes will fail. We urgently need this function now.

[wey-gu]

Dear @bjzhaoqing,
Could you please help create another issue regarding the Ludicrous mode itself, and help us provide more stories on this requirement?

As I understand it correctly, you only need this non-leader readability as a Service degradation instead of concerning about performance.

Thanks!

[wey-gu]

Created #3903