mds:  Don't zap same name till tick

If unique names are configured (as they are by default)
when a new mds sends a beacon, we currently zap any mds
with the same name.  Since the new mds isn't added to the
mdsmap's up list until tick(), this causes a race where a client can
request an mdsmap with no up mds members in the map.  Instead
of zapping at beacon receive, this patch zaps the old MDSs with
the same name when the new mds is added to the up list in tick().
Fixes #4390.

Signed-off-by: Sam Lang <sam.lang@inktank.com>