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>

polkadot/Cargo.lock

@@ -4932,7 +4932,9 @@ version = "0.1.0"
 dependencies = [
  "assert_matches",
  "derive_more 0.99.11",
+ "env_logger",
  "futures 0.3.5",
+ "futures-timer 3.0.2",
  "kvdb",
  "kvdb-memorydb",
  "kvdb-rocksdb",
@@ -4944,6 +4946,7 @@ dependencies = [
  "polkadot-node-subsystem-util",
  "polkadot-overseer",
  "polkadot-primitives",
+ "smallvec 1.4.2",
  "sp-core",
 ]