mon/Elector: bootstrap on timeout

Currently if an election times out we call a new
election.  If we have never joined a quorum, bootstrap
instead. This is heavier weight, but captures the case
where, during bootstrap:

 - a and b have learned each others' addresses
 - everybody calls an election
 - a and b form a quorum
 - c loops trying to call an election, but is ignored
   because a and b don't see its address in the monmap

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