include/CompatSet: make merge const()

With usual shenanigans to deal with [] on map
not being const-safe.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: extract beacon state from MDS lock

For clarify, give it its own class for the
state protected by the separate lock too.

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