mds: warn on unconnected snap realms

When there are more than one active MDS, restarting MDS triggers
assertion "reconnected_snaprealms.empty()" quite often. If there
is no snapshot in the FS, the items left in reconnected_snaprealms
should be other MDS' mdsdir. I think it's harmless.

If there are snapshots in the FS, the assertion probably can catch
real bugs. But at present, snapshot feature is broken, fixing it is
non-trivial. So replace the assertion with a warning.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
(cherry picked from commit 26effc0e583b0a3dade6ec81ef26dec1c94ac8b2)