Collator protocol subsystem (#1659)

* WIP

* The initial implementation of the collator side.

* Improve comments

* Multiple collation requests

* Add more tests and comments to validator side

* Add comments, remove dead code

* Apply suggestions from code review

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

* Fix build after suggested changes

* Also connect to the next validator group

* Remove a Future impl and move TimeoutExt to util

* Minor nits

* Fix build

* Change FetchCollations back to FetchCollation

* Try this

* Final fixes

* Fix build

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

polkadot/node/subsystem/src/messages.rs

@@ -160,7 +160,7 @@ pub enum CollatorProtocolMessage {
 	/// Provide a collation to distribute to validators.
 	DistributeCollation(CandidateReceipt, PoV),
 	/// Fetch a collation under the given relay-parent for the given ParaId.
-	FetchCollation(Hash, ParaId, oneshot::Sender<(CandidateReceipt, PoV)>),
+	FetchCollation(Hash, CollatorId, ParaId, oneshot::Sender<(CandidateReceipt, PoV)>),
 	/// Report a collator as having provided an invalid collation. This should lead to disconnect
 	/// and blacklist of the collator.
 	ReportCollator(CollatorId),
@@ -176,7 +176,7 @@ impl CollatorProtocolMessage {
 		match self {
 			Self::CollateOn(_) => None,
 			Self::DistributeCollation(receipt, _) => Some(receipt.descriptor().relay_parent),
-			Self::FetchCollation(relay_parent, _, _) => Some(*relay_parent),
+			Self::FetchCollation(relay_parent, _, _, _) => Some(*relay_parent),
 			Self::ReportCollator(_) => None,
 			Self::NoteGoodCollation(_) => None,
 			Self::NetworkBridgeUpdateV1(_) => None,