mon/OSDMonitor: update creating_pgs using pending_creatings

mon/OSDMonitor: update creating_pgs using pending_creatings

* without this change, in OSDMonitor::update_pending_pgs(), creating_pgs
  is updated directly without lock, and the change will be overwritten
  with pending changes eventually. so we should update it using
  pending_creatings
* also, should acquire the lock in OSDMonitor::send_pg_creates() when
  reading creating_pgs_by_osd_epoch and createing_pgs.

Fixes: http://tracker.ceph.com/issues/19814
Signed-off-by: Kefu Chai <kchai@redhat.com>