MDSMap:: constify a bunch of methods

Signed-off-by: Sage Weil <sage@inktank.com>
MDSMap: improve health check

Note if the cluster is degraded.  If so, indicate specifically which MDSs
are degraded and what state they are in.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #143 from ceph/wip-mds-health

improve mds health checks

Reviewed-by: Sam Lang <sam.lang@inktank.com>