Implement PoV Distribution Subsystem (#1344)

* introduce candidatedescriptor type

* add PoVDistribution message type

* loosen bound on PoV Distribution to account for equivocations

* re-export some types from the messages module

* begin PoV Distribution subsystem

* remove redundant index from PoV distribution

* define state machine for pov distribution

* handle overseer signals

* set up control flow

* remove `ValidatorStatement` section

* implement PoV fetching

* implement distribution logic

* add missing `

* implement some network bridge event handlers

* stub for message processing, handle our view change

* control flow for handling messages

* handle `awaiting` message

* handle any incoming PoVs and redistribute

* actually provide a subsystem implementation

* remove set-builder notation

* begin testing PoV distribution

* test that we send awaiting messages only to peers with same view

* ensure we distribute awaited PoVs to peers on view changes

* test that peers can complete fetch and are rewarded

* test some reporting logic

* ensure peer is reported for flooding

* test punishing peers diverging from awaited protocol

* test that we eagerly complete peers' awaited PoVs based on what we receive

* test that we prune the awaited set after receiving

* expand pov-distribution in guide to match a change I made

* remove unneeded import

Cargo.toml

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

node/network/README.md

@@ -1 +1 @@
-Stub - This folder will hold networking subsystem implementations, each with their own crate.
+This folder holds all networking subsystem implementations, each with their own crate.

node/network/pov-distribution/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "polkadot-pov-distribution"
+version = "0.1.0"
+authors = ["Parity Technologies <[email protected]>"]
+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"
+sc-network = { git = "https://github.com/paritytech/substrate", branch = "master" }
+sp-runtime = { git = "https://github.com/paritytech/substrate", branch = "master" }
+polkadot-subsystem = { package = "polkadot-node-subsystem", path = "../../subsystem" }
+
+[dev-dependencies]
+parking_lot = "0.10.0"
+subsystem-test = { package = "polkadot-subsystem-test-helpers", path = "../../test-helpers/subsystem" }
+assert_matches = "1.3.0"