mon/PGMonitor: avoid duplicating map_pg_create() effort on same maps

If we have an election and refresh, but the osdmap does not change, there
is no need to recalculate the pg create maps.  However, if we register new
creating pgs, we do... when the last_pg_scan update gets pulled out of
paxos (i.e., on both leader and peon mons).

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