These assertions catch state changes of an mds in standby_daemons to a state
other than MDSMap::STATE_STANDBY. Currently this invariant is (sometimes!)
checked in other locations on access of standby_daemons. This commit allows us
catch the violated invariant at the time it occurred.
Related to: http://tracker.ceph.com/issues/16592
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>