mds: fix handling of initial MDS states

mds: fix handling of initial MDS states

Few things here:

- Make explicit the check for getting removed from the MDSMap. This was
  only done before by checking if MDS held a rank which does not check the
  case where a standby is removed from the FSMap.

- Use mds_info_t::dump to simplify various debug output.

- Add a few sanity asserts for invalid state transitions.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit 26a08df2adc7495fc660113cd26c33d5debd3ee6)

 Conflicts:
	src/mds/MDSDaemon.h

Nautilus still uses "const_ref" rather than "cref_t<>" in master.