Plumb polkadot backend into cumulus-collator (#269)

* The problem in the nutshell

* Make it compile

* make tests pass

* Update the parachain-upgrade module

* Fix collator tests

Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/rococo-parachains/src/service.rs

@@ -187,6 +187,8 @@ where
 		);
 		let spawner = task_manager.spawn_handle();
 
+		let polkadot_backend = polkadot_full_node.backend.clone();
+
 		let params = StartCollatorParams {
 			para_id: id,
 			block_import: client.clone(),
@@ -200,6 +202,7 @@ where
 			polkadot_full_node,
 			spawner,
 			backend,
+			polkadot_backend,
 		};
 
 		start_collator(params).await?;