mon/PGMonitor: always send pg creations after mapping

At some point in the dumpling cycle I separated the map stage from the
send stage, but it is not clear to me now why that is useful.  In
particular, in this particular we can send the creates any time we have
a non-zero osdmap epoch, and are in good shape as long as we do the map
step after the osdmap is loaded (hence the post_paxos_update).

This particular path is responsible for the stalled test referenced in
bug #6673.

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