mon/PGMonitor: revamp how pg creates are tracked

mon/PGMonitor: revamp how pg creates are tracked

Previously we were calculating and managing in-core state that
wasn't committed as part of the pg_map, leading to all sorts of
ugliness that didn't really work.  Instead,

 * set mapping in all creating pgs in the committed pg_map
 * make all pg create message sending be based on committed state
 * update mappings for creating pgs every time we consume a new
   osdmap, so that we have a reliable/stable epoch to attach to
   it.

In particular, having that stable epoch means we have a reference
we can put in the pg create message that will also be used for
the subscription version.  That way OSDs get consistent creates
from any mon.

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