raft: persist new term and vote separately

If a node persisted a foreign term + vote request at the same
time, it increased split-brain probability. A node could vote for
a candidate having smaller vclock than the local one. For example,
via the following scenario:

- Node1, node2, node3 are started;
- Node1 becomes a leader;
- The topology becomes node1 <-> node2 <-> node3 due to network
    issues;
- Node1 sends a synchro txn to node2. The txn starts a WAL write;
- Node3 bumps term and votes for self. Sends it all to node2;
- Node2 votes for node3, because their vclocks are equal;
- Node2 finishes all pending WAL writes, including the txn from
    node1. Now its vclock is > node3's one and the vote was wrong.
- Node3 wins, writes PROMOTE, and it conflicts with node1 writing
    CONFIRM.

This patch makes so a node can't persist a vote in a new term in
the same WAL write as the term bump. Term bump is written first
and alone. It serves as a WAL sync after which the node's vclock
is not supposed to change except for the 0 (local) component.

The vote requests are re-checked after term bump is persisted to
see if they still can be applied.

Part of #7253

NO_DOC=bugfix

(cherry picked from commit c9155ac8)