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

polkadot/primitives/src/parachain.rs

@@ -460,6 +460,17 @@ impl AbridgedCandidateReceipt {
 			pov_block_hash: *pov_block_hash,
 		}
 	}
+
+	/// Clone the relevant portions of the `AbridgedCandidateReceipt` to form a `CandidateDescriptor`.
+	pub fn to_descriptor(&self) -> CandidateDescriptor {
+		CandidateDescriptor {
+			para_id: self.parachain_index,
+			relay_parent: self.relay_parent,
+			collator: self.collator.clone(),
+			signature: self.signature.clone(),
+			pov_hash: self.pov_block_hash.clone(),
+		}
+	}
 }
 
 
@@ -478,6 +489,26 @@ impl Ord for AbridgedCandidateReceipt {
 	}
 }
 
+/// A unique descriptor of the candidate receipt, in a lightweight format.
+#[derive(PartialEq, Eq, Clone, Encode, Decode)]
+#[cfg_attr(feature = "std", derive(Debug, Default))]
+pub struct CandidateDescriptor<H = Hash> {
+	/// The ID of the para this is a candidate for.
+	pub para_id: Id,
+	/// The hash of the relay-chain block this should be executed in
+	/// the context of.
+	// NOTE: the fact that the hash includes this value means that code depends
+	// on this for deduplication. Removing this field is likely to break things.
+	pub relay_parent: H,
+	/// The collator's relay-chain account ID
+	pub collator: CollatorId,
+	/// Signature on blake2-256 of components of this receipt:
+	/// The para ID, the relay parent, and the pov_hash.
+	pub signature: CollatorSignature,
+	/// The hash of the pov-block.
+	pub pov_hash: H,
+}
+
 /// A collation sent by a collator.
 #[derive(PartialEq, Eq, Clone, Encode, Decode)]
 #[cfg_attr(feature = "std", derive(Debug, Default))]