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

Cargo.toml

@@ -45,6 +45,7 @@ members = [
 
 	"node/core/proposer",
 	"node/network/bridge",
+	"node/network/statement-distribution",
 	"node/overseer",
 	"node/primitives",
 	"node/service",

node/network/statement-distribution/Cargo.toml

@@ -0,0 +1,26 @@
+[package]
+name = "polkadot-statement-distribution"
+version = "0.1.0"
+authors = ["Parity Technologies <[email protected]>"]
+description = "Statement Distribution Subsystem"
+edition = "2018"
+
+[dependencies]
+futures = "0.3.5"
+log = "0.4.8"
+futures-timer = "3.0.2"
+streamunordered = "0.5.1"
+polkadot-primitives = { path = "../../../primitives" }
+node-primitives = { package = "polkadot-node-primitives", path = "../../primitives" }
+parity-scale-codec = "1.3.0"
+sp-runtime = { git = "https://github.com/paritytech/substrate", branch = "master" }
+sp-staking = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+polkadot-subsystem = { package = "polkadot-node-subsystem", path = "../../subsystem" }
+arrayvec = "0.5.1"
+indexmap = "1.4.0"
+
+[dev-dependencies]
+parking_lot = "0.10.0"
+subsystem-test = { package = "polkadot-subsystem-test-helpers", path = "../../test-helpers/subsystem" }
+assert_matches = "1.3.0"
+sp-keyring = { git = "https://github.com/paritytech/substrate", branch = "master" }