pgmon: cannot guarantee new PGs have correct OSDMap creation epochs
Following on from #1357:
Right now, pools in the OSDMap track their "last_changed" epoch. This is currently set when the pool is created and never changed.
However, pools can get expanded to include more PGs. Currently this doesn't impact the pool's metadata (apart from pg_num) at all. So when the PGMonitor goes through to create the new PGs, it doesn't have a reliable indicator of what epoch they were created in. :(