mon: fix scrub vs paxos race

Consider:

 - paxos starts a commit N+1
 - a majority of the peers ack it
  - paxos::commit() writes N+1 it to disk
  - tells peers to commit
 - peers commit N+1, *and* refresh_from_paxos(), and generate N+1 full map
 - leader does _scrub on N+1, without latest full osdmap
 - peers do _scrub on N+1, with latest full osdmap
 - leader finishes paxos gather, does refresh_from_paxos()
 -> scrub fails.

Fix this by doing the refresh_from_paxos() at commit time and not when the
paxos round finishes.  This keeps on-disk state more tightly in sync with
in-memory state and avoids the need for a e.g., kludgey workaround in
the scrub code.

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