Availability store pruning (#1820)

* Initial commit

* Move tests to separate module

* Move timestamps to the newtype

* Change idx name

* Use Duration for consts and update chunk records

* Ordering::Equal

* Update node/core/av-store/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* put_ methods do the array sorting

* Fix get_block_number

* Change StoreChunk message type and relay parent method

* Add chunk tests

* Fix block number computation for StoreChunk

* Duration instead of u64 everywhere

* Add a clarifying comment

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>