implement candidate selection subsystem (#1645)

* choose the straightforward candidate selection algorithm for now

* add draft implementation of candidate selection

* fix typo in summary

* more properly report misbehaving collators

* describe how CandidateSelection subsystem becomes aware of candidates

* revise candidate selection / collator protocol interaction pattern

* implement rest of candidate selection per the guide

* review: resolve nits

* start writing test suite, harness

* implement first test

* add second test

* implement third test

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

polkadot/node/subsystem/src/messages.rs

@@ -47,6 +47,8 @@ pub struct NewBackedCandidate(pub BackedCandidate);
 /// Messages received by the Candidate Selection subsystem.
 #[derive(Debug)]
 pub enum CandidateSelectionMessage {
+	/// A candidate collation can be fetched from a collator and should be considered for seconding.
+	Collation(Hash, ParaId, CollatorId),
 	/// We recommended a particular candidate to be seconded, but it was invalid; penalize the collator.
 	/// The hash is the relay parent.
 	Invalid(Hash, CandidateReceipt),
@@ -56,6 +58,7 @@ impl CandidateSelectionMessage {
 	/// If the current variant contains the relay parent hash, return it.
 	pub fn relay_parent(&self) -> Option<Hash> {
 		match self {
+			Self::Collation(hash, ..) => Some(*hash),
 			Self::Invalid(hash, _) => Some(*hash),
 		}
 	}