Request based availability distribution (#2423)

* WIP

* availability distribution, still very wip.

Work on the requesting side of things.

* Some docs on what I intend to do.

* Checkpoint of session cache implementation

as I will likely replace it with something smarter.

* More work, mostly on cache

and getting things to type check.

* Only derive MallocSizeOf and Debug for std.

* availability-distribution: Cache feature complete.

* Sketch out logic in `FetchTask` for actual fetching.

- Compile fixes.
- Cleanup.

* Format cleanup.

* More format fixes.

* Almost feature complete `fetch_task`.

Missing:

- Check for cancel
- Actual querying of peer ids.

* Finish FetchTask so far.

* Directly use AuthorityDiscoveryId in protocol and cache.

* Resolve `AuthorityDiscoveryId` on sending requests.

* Rework fetch_task

- also make it impossible to check the wrong chunk index.
- Export needed function in validator_discovery.

* From<u32> implementation for `ValidatorIndex`.

* Fixes and more integration work.

* Make session cache proper lru cache.

* Use proper lru cache.

* Requester finished.

* ProtocolState -> Requester

Also make sure to not fetch our own chunk.

* Cleanup + fixes.

* Remove unused functions

- FetchTask::is_finished
- SessionCache::fetch_session_info

* availability-distribution responding side.

* Cleanup + Fixes.

* More fixes.

* More fixes.

adder-collator is running!

* Some docs.

* Docs.

* Fix reporting of bad guys.

* Fix tests

* Make all tests compile.

* Fix test.

* Cleanup + get rid of some warnings.

* state -> requester

* Mostly doc fixes.

* Fix test suite.

* Get rid of now redundant message types.

* WIP

* Rob's review remarks.

* Fix test suite.

* core.relay_parent -> leaf for session request.

* Style fix.

* Decrease request timeout.

* Cleanup obsolete errors.

* Metrics + don't fail on non fatal errors.

* requester.rs -> requester/mod.rs

* Panic on invalid BadValidator report.

* Fix indentation.

* Use typed default timeout constant.

* Make channel size 0, as each sender gets one slot anyways.

* Fix incorrect metrics initialization.

* Fix build after merge.

* More fixes.

* Hopefully valid metrics names.

* Better metrics names.

* Some tests that already work.

* Slightly better docs.

* Some more tests.

* Fix network bridge test.