Introduce CollectCollationInfo runtime api (#443)

* Introduce `CollectCollationInfo` runtime api

Instead of using well known keys to communicate information about a
collation between the runtime and the collator, we now use a runtime api
for this.

* Fixes bug

* Apply suggestions from code review

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Doc update

Co-authored-by: Sergei Shulepov <sergei@parity.io>

cumulus/polkadot-parachains/src/service.rs

@@ -180,7 +180,8 @@ where
 			Block,
 			StateBackend = sc_client_api::StateBackendFor<TFullBackend<Block>, Block>,
 		> + sp_offchain::OffchainWorkerApi<Block>
-		+ sp_block_builder::BlockBuilder<Block>,
+		+ sp_block_builder::BlockBuilder<Block>
+		+ cumulus_primitives_core::CollectCollationInfo<Block>,
 	sc_client_api::StateBackendFor<TFullBackend<Block>, Block>: sp_api::StateBackend<BlakeTwo256>,
 	Executor: sc_executor::NativeExecutionDispatch + 'static,
 	RB: Fn(
@@ -302,7 +303,6 @@ where
 			collator_key,
 			relay_chain_full_node,
 			spawner,
-			backend,
 			parachain_consensus,
 		};