Fix: crmd: don't fence old DC if it's shutting down as soon-to-be DC joins

Existing peers of a DC that is shutting down can avoid fencing it (by updating
its expected state) because it broadcasts its shutdown request. However, a
newly joining node won't get that broadcast.

Previously, if the joining node became the new DC, it would fence the old one.
Now, the DC notifies joining nodes (via a join message field) whether it is
shutting down, and joining nodes update its expected state accordingly.