mon/MDSMonitor: do not generate mdsmaps from already-laggy mds

There is one path where a mds that is not sending its beacon (e.g.,
because it is not running at all) will lead to proposal of new mdsmaps.
Fix it.

Backport: emperor, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1123 from ceph/wip-stray-mdsmaps

mon/MDSMonitor: do not generate mdsmaps from already-laggy mds
Reviewed-by: Greg Farnum <greg@inktank.com>