mds: promote Beacon to be a Dispatcher

mds: promote Beacon to be a Dispatcher

This allows it to handle its own MSG_MDS_BEACON
messages from the mon, outside of mds_lock.

This is less important than the sending of beacons
being outside the lock, but still nice to have and
gets all the beacon messaging in one place.

Signed-off-by: John Spray <john.spray@redhat.com>