For asynchronous backing, the candidate-backing subsystem will have a few key changes.

We need to notify the prospective-parachains subsystem about all new seconded candidates and backed candidates. We'll probably remove the concept of seconding two candidates under the same relay-parent being misbehavior (spam should be prevented on the network level - #5055).

And we'll need to keep around statement tables until relay-parents go obsolete, not just inactive. More specifically, we should continue accepting candidates for relay-parents as long as any of the active leaves could consider that relay-parent to be relevant. See #4963