box: split promote() into reasonable parts

box_promote() is a monster. It does a lot of different things based on
flags: try_wait and run_elections. The flags themselves depend on the
node's Raft state and the lunar calendar.

Moreover, there are multiple cancellation points and places where
external state may have changed and needs a re-check.

Things are going to get even worse with the introduction of box.ctl.demote().

So it's time to split up box_promote() into reasonable parts, each doing
exactly one thing.

This commit mostly addresses the multiple cancellation points issue,
so that promote() doesn't look like a huge pile of if(something_changed)
blocks. Some other functions will look like that instead.

Part of #6034