raft: make promote bump term and vote at once

box.ctl.promote() was implemented as follows: an instance bumps the
term and marks itself a candidate, but doesn't vote for self
immediately. Instead it relies on the machinery which makes a candidate
vote for self as soon as it persists a new term.

This differs from a normal election start due to leader timeout: there
term and vote are bumped at once.

Besides, this increases probability of box.ctl.promote() resulting in
other node getting elected: if a node first broadcasts a term without a
vote, it is not considered a candidate, so other candidates might start
elections and vote for themselves.

Let's bring promote into line with automatic elections.

Closes #8497

NO_DOC=bugfix

(cherry picked from commit 17371215)