* Change best effort queue behaviour in `dispute-coordinator`
Use the same type of queue (`BTreeMap<CandidateComparator,
ParticipationRequest>`) for best effort and priority in
`dispute-coordinator`.
Rework `CandidateComparator` to handle unavailable parent
block numbers.
Best effort queue will order disputes the same way as priority does - by
parent's block height. Disputes on candidates for which the parent's
block number can't be obtained will be treated with the lowest priority.
* Fix tests: Handle `ChainApiMessage::BlockNumber` in `handle_sync_queries`
* Some tests are deadlocking on sending messages via overseer so change `SingleItemSink`to `mpsc::Sender` with a buffer of 1
* Fix a race in test after adding a buffered queue for overseer messages
* Fix the rest of the tests
* Guide update - best-effort queue
* Guide update: clarification about spam votes
* Fix tests in `availability-distribution`
* Update comments
* Add `make_buffered_subsystem_context` in `subsystem-test-helpers`
* Code review feedback
* Code review feedback
* Code review feedback
* Don't add best effort candidate if it is already in priority queue
* Remove an old comment
* Fix insert in best_effort