impl approval distribution (#2160)

* initial impl approval distribution

* initial tests and fixes

* batching seems difficult: different peers have different needs

* bridge: fix test after merge

* some guide updates

* only send assignments to peers who know about the block

* fix a test, add approvals test

* simplify

* do not send assignment to peers for finalized blocks

* guide: protocol input and output

* one more test

* more comments, logs, initial metrics

* fix a typo

* one more thing: early return when reimporting a thing locally

polkadot/primitives/src/v1.rs

@@ -150,6 +150,9 @@ mod assigment_app {
 /// to approve included parachain candidates.
 pub type AssignmentId = assigment_app::Public;
 
+/// The index of the candidate in the list of candidates fully included as-of the block.
+pub type CandidateIndex = u32;
+
 /// Get a collator signature payload on a relay-parent, block-data combo.
 pub fn collator_signature_payload<H: AsRef<[u8]>>(
 	relay_parent: &H,
@@ -504,7 +507,7 @@ pub fn check_candidate_backing<H: AsRef<[u8]> + Clone + Encode>(
 }
 
 /// The unique (during session) index of a core.
-#[derive(Encode, Decode, Default, PartialOrd, Ord, Eq, PartialEq, Clone, Copy)]
+#[derive(Encode, Decode, Default, PartialOrd, Ord, Eq, PartialEq, Clone, Copy, Hash)]
 #[cfg_attr(feature = "std", derive(Debug))]
 pub struct CoreIndex(pub u32);