mon: Monitor: suicide on start if mon has been removed from monmap

If the monitor has been marked as having been part of an existing quorum
and is no longer in the monmap, then it is safe to assume the monitor
was removed from the monmap.  In that event, do not allow the monitor
to start, as it will try to find its way into the quorum again (and
someone clearly stated they don't really want them there).

Fixes: 6789
Backport: dumpling, emperor

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>