mon: once sync full is chosen, make sure we don't change our mind

It is possible for a sequence like:

 - probe
 - first probe reply has paxos trim that indicates a full sync is
   needed
 - start sync
 - clear store
 - something happens that makes us abort and bootstrap (e.g., the
   provider mon restarts
 - probe
 - first probe reply has older paxos trim bound and we call an election
 - on election completion, we crash because we have no data.

Non-determinism of the probe decision aside, we need to ensure that
the info we share during probe (fc, lc) is accurate, and that once we
clear the store we know we *must* do a full sync.

This is a backport of aa60f940ec1994a61624345586dc70d261688456.

Fixes: #5621
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>