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>

test/service/src/lib.rs

@@ -266,7 +266,6 @@ where
 			relay_chain_full_node.with_client(polkadot_test_service::TestClient);
 
 		let params = StartCollatorParams {
-			backend: params.backend,
 			block_status: client.clone(),
 			announce_block,
 			client: client.clone(),