dispute-coordinator: Prioritization on best-effort queue is broken with batching

[eskimor]

With the upcoming batching of votes the logic for prioritizing participation requests on the best-effort queue is broken. I bumps up priority based on the number of times a candidate is added to the queue - which currently equals the number of times a vote is imported. With batched imports this is no longer true and needs to be fixed, e.g. by passing in the number of votes that have been imported for the request to happen.

[eskimor]

Even better, with the upcoming change in meaning what best-effort means, we can actually use the same mechanism as the priority queue and order based on block height of the relay parent. best-effort just becomes another instance of the same kind of queue.

[eskimor]

There is one edge case though. We we stick with the ideas in the guide, then we also participate if the dispute got confirmed. In that case we would not know the relay parent. Such participation requests should be treated with the lowest priority.