raft: introduce split vote detection

Split vote is a situation when during election nobody can win and
will not win in this term for sure. Not a single node could get
enough votes. For example, with 4 nodes one could get 2 votes and
other also 2 votes. Nobody will get quorum 3 in this term.

The patch makes raft able to notice that situation and speed up
the term bump. It is not bumped immediately though, because nodes
might do that simultaneously and will get the split vote again
after voting for self. There is a random delay. But it is just max
10% of election timeout, so it should speed up split vote
resolution on 90% at least.

Part of #5285