mds: re-add if standby and removed from mdsmap
mds: re-add if standby and removed from mdsmap

Instead of shutting down, we should stay alive and try to re-add ourselves
to the MDSMap.

This paves the way for the monitor to remove laggy standby nodes instead
of just marking them laggy.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph