Request based PoV distribution (#2640)

* Indentation fix.

* Prepare request-response for PoV fetching.

* Drop old PoV distribution.

* WIP: Fetch PoV directly from backing.

* Backing compiles.

* Runtime access and connection management for PoV distribution.

* Get rid of seemingly dead code.

* Implement PoV fetching.

Backing does not yet use it.

* Don't send `ConnectToValidators` for empty list.

* Even better - no need to check over and over again.

* PoV fetching implemented.

+ Typechecks
+ Should work

Missing:

- Guide
- Tests
- Do fallback fetching in case fetching from seconding validator fails.

* Check PoV hash upon reception.

* Implement retry of PoV fetching in backing.

* Avoid pointless validation spawning.

* Add jaeger span to pov requesting.

* Add back tracing.

* Review remarks.

* Whitespace.

* Whitespace again.

* Cleanup + fix tests.

* Log to log target in overseer.

* Fix more tests.

* Don't fail if group cannot be found.

* Simple test for PoV fetcher.

* Handle missing group membership better.

* Add test for retry functionality.

* Fix flaky test.

* Spaces again.

* Guide updates.

* Spaces.

polkadot/node/network/bridge/src/multiplexer.rs

@@ -141,6 +141,11 @@ fn multiplex_single(
 			decode_with_peer::<v1::CollationFetchingRequest>(peer, payload)?,
 			pending_response,
 		)),
+		Protocol::PoVFetching => From::from(IncomingRequest::new(
+			peer,
+			decode_with_peer::<v1::PoVFetchingRequest>(peer, payload)?,
+			pending_response,
+		)),
 		Protocol::AvailableDataFetching => From::from(IncomingRequest::new(
 			peer,
 			decode_with_peer::<v1::AvailableDataFetchingRequest>(peer, payload)?,