rgw: call update_latest_epoch() on all period updates

rgw: call update_latest_epoch() on all period updates

when updating the period, callers use the atomic result of
update_latest_epoch() to determine whether they need to call
RGWPeriod::reflect() and RGWRealm::notify_new_period()

this adds a missing call to RGWPeriod::reflect() to RGWPeriodPuller,
which was previously not safe to do without atomic updates to
latest_epoch

Fixes: http://tracker.ceph.com/issues/19817
Signed-off-by: Casey Bodley <cbodley@redhat.com>