Validator side of the collation protocol. (#295)

* skeleton of collators object

* awaiting and handling collations. rename `collators` to CollationPool

* add some tests

* add tests

* implement Collators trait for ConsensusNetwork

* plug collators into main polkadot-network

* ignore collator role message

* add a couple more tests

* garbage collection for collations

* ensure disconnected backup collator is removed from pool

* address other grumbles

polkadot/primitives/src/parachain.rs

@@ -144,7 +144,7 @@ pub struct CandidateReceipt {
 	pub parachain_index: Id,
 	/// The collator's relay-chain account ID
 	pub collator: super::AccountId,
-	/// Signature on block data by collator.
+	/// Signature on blake2-256 of the block data by collator.
 	pub signature: CandidateSignature,
 	/// The head-data
 	pub head_data: HeadData,
@@ -195,6 +195,17 @@ impl CandidateReceipt {
 		use runtime_primitives::traits::{BlakeTwo256, Hash};
 		BlakeTwo256::hash_of(self)
 	}
+
+	/// Check integrity vs. provided block data.
+	pub fn check_signature(&self) -> Result<(), ()> {
+		use runtime_primitives::traits::Verify;
+
+		if self.signature.verify(&self.block_data_hash.0[..], &self.collator) {
+			Ok(())
+		} else {
+			Err(())
+		}
+	}
 }
 
 impl PartialOrd for CandidateReceipt {