mds: set resolve/rejoin gather MDS set in advance

mds: set resolve/rejoin gather MDS set in advance

For active MDS, it may receive resolve/rejoin message before receiving
the mdsmap message that claims the MDS cluster is in resolving/rejoning
state. So instead of set the gather MDS set when receiving the mdsmap.
set them in advance when detecting MDS' failure.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Greg Farnum <greg@inktank.com>