provide more secure raft read mechanism

[kikimo]

We skim the raft-based kv store and found that for read operation, there seems only leased read operation to defend against stale read, though good in performance leased read failed to handle situation like wall-clock drifting, for a strong consistant system, we might need to consider more secure raft kv read mechanism like Indexed Read which is implemented by ETCD and used as its defualt option.

[critical27]

Will focus on more serious problem of raft in 3.0