mon: set peon state to electing if other mons call an election

mon: set peon state to electing if other mons call an election

Previously we would call mon->reset() and set various flags (like
exited_quorum timestamp), but the state would remain PEON.  Make an
explicit join_election() callback and set the state there, and add
asserts in reset() (renamed to be private) so that we ensure all
callers are well-behaved.

Signed-off-by: Sage Weil <sage@inktank.com>