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/Cargo.lock

@@ -4831,6 +4831,28 @@ dependencies = [
  "westend-runtime",
 ]
 
+[[package]]
+name = "polkadot-statement-distribution"
+version = "0.1.0"
+dependencies = [
+ "arrayvec 0.5.1",
+ "assert_matches",
+ "futures 0.3.5",
+ "futures-timer 3.0.2",
+ "indexmap",
+ "log 0.4.8",
+ "parity-scale-codec",
+ "parking_lot 0.10.2",
+ "polkadot-node-primitives",
+ "polkadot-node-subsystem",
+ "polkadot-primitives",
+ "polkadot-subsystem-test-helpers",
+ "sp-keyring",
+ "sp-runtime",
+ "sp-staking",
+ "streamunordered",
+]
+
 [[package]]
 name = "polkadot-statement-table"
 version = "0.8.14"