raft: reset votes when becomePreCandidate

[lishuai87]

raft: reset votes when becomePreCandidate. because the poll method wouldn't update grant state when key exists.

here is a case:

1. cluster with three node [A, B, C], all term are 2, with leader A and all logs are replicated.
2. node A down. B and C start election at the same time (split votes), both become pre-candidate.
3. C grant B's pre-vote, and B grant C's pre-vote. So, both B and C become candidate, with term 3.
4. C reject B's vote, and B reject C's vote. So, B and C are still candidate. B record votes like (B, true) (C, false), C record votes like (B, false) (C, true)
5. B election-timeout first, start pre-vote. C grant B's pre-vote.
6. when B check poll state，it's still (B, true) (C, false). So, election will continue, and no one will win.

[xiang90]

need test

[lishuai87]

add a new test case, TestPreVoteWithSplitVote

[xiang90]

check line612? it says vote should not be changed for some reason. i do not feel it is safe to change the vote decision for one term either.

/cc @bdarnell

[xiang90]

probably if pre-candidate times out, it needs to increase term to handle the potential vote spilt.

[lishuai87]

vote is what we voted for, it's in hard state. while votes is what other nodes grant/reject us, it's a memory state.
we can't change vote, but can change votes when a pre-vote happen.

[xiang90]

Right. I misread.

[xiang90]

Lgtm