Availability Store should use determine_new_blocks on import notifications

[rphmeier]

These logs: "Candidate included without being backed?", can stem from missing block import notifications and CandidateEvent::Backed. Missing these and other candidate events will lead to storage leaks over time as well.

#3261 implements a utility for backwards iteration, so it should be easy to plug this into the availability store now.

The only thing is that it relies on both a lower bound number and an is_known function which might need some changes in the av-store schema.