mon/OSDMonitor: transit creating_pgs from pgmap when upgrading
there could be some pg(s) still being created when we are upgrading to
luminous, and the pools holding them are not changed in the sense of
pg_pool_t::last_change after the upgrade and before we scan for
creating pgs. in that case, the existing update_pending_creatings()
will fail to collect the pgs being created before the upgrade.
with this change, the creating_pgs in pgmap are also used for updating
the OSDMonitor's creating_pgs if it's updated.
but we should stopupdating the pgmap once the upgrade completes. i.e.
stop dispatching MSG_PGSTATS messages to PGMonitor if the quorum and all
osds are luminous.