Learner should respond to request vote but don't accept votes

[BusyJay]

Consider following case, a cluster has three nodes A, B, C, and A is Leader. And then add learner D, E to the cluster. D, E is promoted to voter then, but D is isolated from A, so D doesn't know himself is promoted. When A, B both crash, the cluster still has three voters C, D, E, which should be able to form a quorum. However, D can't vote in the current implementation, since it doesn't know it's not learner anymore.

One solution to this problem is let learner respond to vote request and let candidate check if the response is from a valid peer. Because we won't let a voter step back as learner, so when candidate consider a node is a voter, then it's definitely a voter from that point to infinite future. So the candidate check should be safe. In the situation described above, the quorum can be formed again, and the cluster can recover automatically.

/cc @xiang90 @siddontang any thoughts?

[siddontang]

PTAL @xiang90 @Hoverbear