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>

polkadot/node/subsystem-types/src/messages.rs

@@ -830,8 +830,10 @@ pub enum ProvisionerMessage {
 /// Message to the Collation Generation subsystem.
 #[derive(Debug)]
 pub enum CollationGenerationMessage {
-	/// Initialize the collation generation subsystem
+	/// Initialize the collation generation subsystem.
 	Initialize(CollationGenerationConfig),
+	/// Reinitialize the collation generation subsystem, overriding the existing config.
+	Reinitialize(CollationGenerationConfig),
 	/// Submit a collation to the subsystem. This will package it into a signed
 	/// [`CommittedCandidateReceipt`] and distribute along the network to validators.
 	///