mon/PGMap: update osd_epoch in synchrony with osd_stat_updates

mon/PGMap: update osd_epoch in synchrony with osd_stat_updates

I'm not sure why this didn't bite us earlier, but there is an assert
in apply_incremental (not used in preluminous mon) and an implicit
dereference in PGMonitor::encode_pending (maybe didn't cause crash?)
that will trigger if we have an osd_stat_updates record without a matching
osd_epochs update.  Maybe there is some subtle reason why the osd_epochs
update happens elsewhere in master (it doesn't on the mgr), but my guess
is we were silently dereferencing the invalid iterator and not noticing.

Anyway, it's easy to fix.  We use the epoch from the previous PGMap.

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