mon/MonClient: set configs via finisher

mon/MonClient: set configs via finisher

The config observers may want to take locks that are ordered relative
to monc_lock.

We could simply drop monc_lock for this call, but that would implicitly
rely on a single-threaded dispatch to avoid having two incoming MConfig
messages get reordered.  Explicitly putting it on a finisher is safer.
Note that we adjust the get_monmap_and_config() to start, drain, and stop
the finisher to ensure we have incoming config processed and applied
before returning.

Fixes: http://tracker.ceph.com/issues/24118
Signed-off-by: Sage Weil <sage@redhat.com>