mon/PGMonitor: always send pg creations after mapping

At some point in the dumpling cycle I separated the map stage from the
send stage.  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).

Some background:

We originally introduced the map-but-don't send in a2fe0137, at which
point all was well because we only called it on ceph-mon startup.

Later, this turned into post_paxos_update in e635c478, at which point
it was now called by a running monitor.. but we didn't add in the
send_pg_creates().  This is where this bug stems from.

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

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