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>
(cherry picked from commit 584c2dd6bea3fe1a3c7f306874c054ce0cf0d2b5)