Make CandidateHash a real type (#1916)

* Make `CandidateHash` a real type

This pr adds a new type `CandidateHash` that is used instead of the
opaque `Hash` type. This helps to ensure on the type system level that
we are passing the correct types.

This pr also fixes wrong usage of `relay_parent` as `candidate_hash`
when communicating with the av storage.

* Update core-primitives/src/lib.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Wrap the lines

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

polkadot/node/network/protocol/src/lib.rs

@@ -186,7 +186,7 @@ impl View {
 pub mod v1 {
 	use polkadot_primitives::v1::{
 		Hash, CollatorId, Id as ParaId, ErasureChunk, CandidateReceipt,
-		SignedAvailabilityBitfield, PoV,
+		SignedAvailabilityBitfield, PoV, CandidateHash,
 	};
 	use polkadot_node_primitives::SignedFullStatement;
 	use parity_scale_codec::{Encode, Decode};
@@ -198,7 +198,7 @@ pub mod v1 {
 	pub enum AvailabilityDistributionMessage {
 		/// An erasure chunk for a given candidate hash.
 		#[codec(index = "0")]
-		Chunk(Hash, ErasureChunk),
+		Chunk(CandidateHash, ErasureChunk),
 	}
 
 	/// Network messages used by the bitfield distribution subsystem.