Raft votes need to advertise their commit_idx instead of their last_idx

[olgavrou]

When there is an election in Raft, nodes currently send their last_idx to indicate their state. Nodes compare their last_idxs to decide whether to vote for a candidate or not.

This can lead to nodes being elected even though they are further behind. If node (A) is at last_idx 2 but commit_idx 1 and node (B) is at last_idx 2 and commit_idx 2, then node (A) can win an election even though it will immediately after rollback to it's commit_idx 1, leaving the follower ahead of the leader.

This can be fixed by using commit_idx instead of last_idx to vote.

[achamayou]

No impact other than performance effectively (the window of loss on election is greater than it could be).