mon/Paxos: make backend write async

mon/Paxos: make backend write async

Move into the WRITING state and do the write to leveldb (or whatever the
backend is) asynchronously.

A few tricks here:
 - we can't do the is_updating() state check because we will always be in
   REFRESH.  Instead, make commit_proposal() tolerate the case where it is
   called but the top proposal isn't the one we just did (or the list is
   empty).  This makes the callers simpler.
 - do_refresh() may call bootstrap.  If we do bootstrap while in REFRESH,
   don't do a sync/flush on the backend store because *we* are async
   completion thread and we'll deadlock.  All other callers need to wait
   for this, though!

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