alternate availability store schema (#2237)

* alternate availability store schema

* improvements

* tweaks

* new DB schema and skeleton

* expand skeleton and tweaks

* handle backing and inclusion

* let finality be handled later

* handle finalized blocks

* implement query methods

* implement chunk storing

* StoreAvailableData

* fix an off-by-one

* implement pruning

* reinstate subsystem trait impl

* reinstate metrics

* fix warnings

* remove chunks_cache

* oops

* actually store the available data

* mockable pruning interval

* fix tests

* spacing

* fix code grumbles

* guide improvements

* make time mockable

* implement a mocked clock for testing

* return DB errors

* Update node/core/av-store/Cargo.toml

Co-authored-by: Bastian Köcher <[email protected]>

* Update roadmap/implementers-guide/src/node/utility/availability-store.md

Co-authored-by: Bastian Köcher <[email protected]>

* Update roadmap/implementers-guide/src/node/utility/availability-store.md

Co-authored-by: Bastian Köcher <[email protected]>

* review grumbles & clarity

* fix review grumbles

* Add docs

Co-authored-by: Andronik Ordian <[email protected]>

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Andronik Ordian <[email protected]>