Implement the Statement Distribution Subsystem (#1326)

* set up data types and control flow for statement distribution

* add some set-like methods to View

* implement sending to peers

* start fixing equivocation handling

* Add a section to the statement distribution subsystem on equivocations and flood protection

* fix typo and amend wording

* implement flood protection

* have peer knowledge tracker follow when peer first learns about a candidate

* send dependents after circulating

* add another TODO

* trigger send in one more place

* refactors from review

* send new statements to candidate backing

* instantiate active head data with runtime API values

* track our view changes and peer view changes

* apply a benefit to peers who send us statements we want

* remove unneeded TODO

* add some comments and improve Hash implementation

* start tests and fix `note_statement`

* test active_head seconding logic

* test that the per-peer tracking logic works

* test per-peer knowledge tracker

* test that peer view updates lead to messages being sent

* test statement circulation

* address review comments

* have view set methods return references

polkadot/primitives/src/parachain.rs

@@ -595,7 +595,7 @@ pub struct Activity(#[cfg_attr(feature = "std", serde(with="bytes"))] pub Vec<u8
 
 /// Statements that can be made about parachain candidates. These are the
 /// actual values that are signed.
-#[derive(Clone, PartialEq, Eq, Encode, Decode)]
+#[derive(Clone, PartialEq, Eq, Encode, Decode, Hash)]
 #[cfg_attr(feature = "std", derive(Debug))]
 pub enum CompactStatement {
 	/// Proposal of a parachain candidate.
@@ -609,6 +609,18 @@ pub enum CompactStatement {
 	Invalid(Hash),
 }
 
+impl CompactStatement {
+	/// Get the underlying candidate hash this references.
+	pub fn candidate_hash(&self) -> &Hash {
+		match *self {
+			CompactStatement::Candidate(ref h)
+				| CompactStatement::Valid(ref h)
+				| CompactStatement::Invalid(ref h)
+				=> h
+		}
+	}
+}
+
 /// A signed compact statement, suitable to be sent to the chain.
 pub type SignedStatement = Signed<CompactStatement>;