Enable async backing on asset-hub-rococo (#2826)

The goal is to move all system chains on Rococo (+ other testnets?) to
use async backing.

Starting with `asset-hub-rococo` to get feedback, before I do the rest.

## Related

Example: https://github.com/paritytech/polkadot-sdk/pull/1619/
Guide:
https://github.com/w3f/polkadot-wiki/blob/master/docs/maintain/maintain-guides-async-backing.md

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>

cumulus/client/collator/src/lib.rs

@@ -242,17 +242,19 @@ pub async fn initialize_collator_subsystems(
 	overseer_handle: &mut OverseerHandle,
 	key: CollatorPair,
 	para_id: ParaId,
+	reinitialize: bool,
 ) {
-	overseer_handle
-		.send_msg(
-			CollationGenerationMessage::Initialize(CollationGenerationConfig {
-				key,
-				para_id,
-				collator: None,
-			}),
-			"StartCollator",
-		)
-		.await;
+	let config = CollationGenerationConfig { key, para_id, collator: None };
+
+	if reinitialize {
+		overseer_handle
+			.send_msg(CollationGenerationMessage::Reinitialize(config), "StartCollator")
+			.await;
+	} else {
+		overseer_handle
+			.send_msg(CollationGenerationMessage::Initialize(config), "StartCollator")
+			.await;
+	}
 
 	overseer_handle
 		.send_msg(CollatorProtocolMessage::CollateOn(para_id), "StartCollator")