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/node/subsystem/src/messages.rs

@@ -24,7 +24,6 @@
 
 use futures::channel::{mpsc, oneshot};
 
-use sc_network::{ObservedRole, ReputationChange, PeerId};
 use polkadot_primitives::{BlockNumber, Hash, Signature};
 use polkadot_primitives::parachain::{
 	AbridgedCandidateReceipt, PoVBlock, ErasureChunk, BackedCandidate, Id as ParaId,
@@ -34,6 +33,8 @@ use polkadot_node_primitives::{
 	MisbehaviorReport, SignedFullStatement, View, ProtocolId,
 };
 
+pub use sc_network::{ObservedRole, ReputationChange, PeerId};
+
 /// A notification of a new backed candidate.
 #[derive(Debug)]
 pub struct NewBackedCandidate(pub BackedCandidate);
@@ -234,4 +235,6 @@ pub enum AllMessages {
 	RuntimeApi(RuntimeApiMessage),
 	/// Message for the availability store subsystem.
 	AvailabilityStore(AvailabilityStoreMessage),
+	/// Message for the network bridge subsystem.
+	NetworkBridge(NetworkBridgeMessage),
 }