mon: explicitly refresh_from_paxos() when leveldb state changes

mon: explicitly refresh_from_paxos() when leveldb state changes

Instead of opportunistically calling each service's update_from_paxos(),
instead explicitly refresh all in-memory state whenever we know the
paxos state may have changed.  This is simpler and less fragile.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit cc339c07312006e65854207523f50542d00ecf87)