* Apply suggestions from #1364 code review
- use CoreState, not CoreOccupied
- query for availability chunks, not the whole PoV
- create a stub `fn availability_cores`
* link to issue documenting unimplemented
* implement get_availability_cores by adding a new runtime api request
* back out an unrelated change properly part of #1404
* av-store: handle QueryChunkAvailability
* simplify QueryDataAvailability
* remove extraneous whitespace
* compact primitive imports