mon/OSDMonitor: use newest creation epoch for pgs that we can

mon/OSDMonitor: use newest creation epoch for pgs that we can

If we have a huge pool it may take a while for the PGs to get out of the
queue and be created.  If we use the epoch the pool was created it may
mean a lot of old OSDMaps the OSD has to process.  If we use the current
epoch (the first epoch in which any OSD learned that this PG should
exist) we limit PastIntervals as much as possible.

It is still possible that we start trying to create a PG but the cluster
is unhealthy for a long time, resulting in a long PastIntervals that
needs to be generated by a primary OSD when it eventually comes up. So
this only partially

Partially-fixes: http://tracker.ceph.com/issues/20050
Signed-off-by: Sage Weil <sage@redhat.com>