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/parachains/runtimes/bridge-hubs/test-utils/src/test_cases/from_grandpa_chain.rs

@@ -39,7 +39,7 @@ use bridge_runtime_common::{
 use frame_support::traits::{Get, OnFinalize, OnInitialize};
 use frame_system::pallet_prelude::BlockNumberFor;
 use parachains_runtimes_test_utils::{
-	AccountIdOf, BasicParachainRuntime, CollatorSessionKeys, RuntimeCallOf,
+	AccountIdOf, BasicParachainRuntime, CollatorSessionKeys, RuntimeCallOf, SlotDurations,
 };
 use sp_keyring::AccountKeyring::*;
 use sp_runtime::{traits::Header as HeaderT, AccountId32};
@@ -107,6 +107,7 @@ where
 /// Also verifies relayer transaction signed extensions work as intended.
 pub fn relayed_incoming_message_works<RuntimeHelper>(
 	collator_session_key: CollatorSessionKeys<RuntimeHelper::Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	bridged_chain_id: bp_runtime::ChainId,
 	sibling_parachain_id: u32,
@@ -136,6 +137,7 @@ pub fn relayed_incoming_message_works<RuntimeHelper>(
 		RuntimeHelper::MPI,
 	>(
 		collator_session_key,
+		slot_durations,
 		runtime_para_id,
 		sibling_parachain_id,
 		local_relay_chain_id,
@@ -205,6 +207,7 @@ pub fn relayed_incoming_message_works<RuntimeHelper>(
 /// Also verifies relayer transaction signed extensions work as intended.
 pub fn complex_relay_extrinsic_works<RuntimeHelper>(
 	collator_session_key: CollatorSessionKeys<RuntimeHelper::Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	sibling_parachain_id: u32,
 	bridged_chain_id: bp_runtime::ChainId,
@@ -237,6 +240,7 @@ pub fn complex_relay_extrinsic_works<RuntimeHelper>(
 		RuntimeHelper::MPI,
 	>(
 		collator_session_key,
+		slot_durations,
 		runtime_para_id,
 		sibling_parachain_id,
 		local_relay_chain_id,

cumulus/parachains/runtimes/bridge-hubs/test-utils/src/test_cases/from_parachain.rs

@@ -40,7 +40,7 @@ use bridge_runtime_common::{
 use frame_support::traits::{Get, OnFinalize, OnInitialize};
 use frame_system::pallet_prelude::BlockNumberFor;
 use parachains_runtimes_test_utils::{
-	AccountIdOf, BasicParachainRuntime, CollatorSessionKeys, RuntimeCallOf,
+	AccountIdOf, BasicParachainRuntime, CollatorSessionKeys, RuntimeCallOf, SlotDurations,
 };
 use sp_keyring::AccountKeyring::*;
 use sp_runtime::{traits::Header as HeaderT, AccountId32};
@@ -112,6 +112,7 @@ where
 /// Also verifies relayer transaction signed extensions work as intended.
 pub fn relayed_incoming_message_works<RuntimeHelper>(
 	collator_session_key: CollatorSessionKeys<RuntimeHelper::Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	bridged_para_id: u32,
 	bridged_chain_id: bp_runtime::ChainId,
@@ -146,6 +147,7 @@ pub fn relayed_incoming_message_works<RuntimeHelper>(
 		RuntimeHelper::MPI,
 	>(
 		collator_session_key,
+		slot_durations,
 		runtime_para_id,
 		sibling_parachain_id,
 		local_relay_chain_id,
@@ -244,6 +246,7 @@ pub fn relayed_incoming_message_works<RuntimeHelper>(
 /// Also verifies relayer transaction signed extensions work as intended.
 pub fn complex_relay_extrinsic_works<RuntimeHelper>(
 	collator_session_key: CollatorSessionKeys<RuntimeHelper::Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	bridged_para_id: u32,
 	sibling_parachain_id: u32,
@@ -281,6 +284,7 @@ pub fn complex_relay_extrinsic_works<RuntimeHelper>(
 		RuntimeHelper::MPI,
 	>(
 		collator_session_key,
+		slot_durations,
 		runtime_para_id,
 		sibling_parachain_id,
 		local_relay_chain_id,

cumulus/parachains/runtimes/bridge-hubs/test-utils/src/test_cases/helpers.rs

@@ -31,7 +31,7 @@ use frame_system::pallet_prelude::BlockNumberFor;
 use pallet_bridge_grandpa::{BridgedBlockHash, BridgedHeader};
 use parachains_common::AccountId;
 use parachains_runtimes_test_utils::{
-	mock_open_hrmp_channel, AccountIdOf, CollatorSessionKeys, RuntimeCallOf,
+	mock_open_hrmp_channel, AccountIdOf, CollatorSessionKeys, RuntimeCallOf, SlotDurations,
 };
 use sp_core::Get;
 use sp_keyring::AccountKeyring::*;
@@ -220,6 +220,7 @@ pub fn relayer_id_at_bridged_chain<Runtime: pallet_bridge_messages::Config<MPI>,
 /// with proofs (finality, message) independently submitted.
 pub fn relayed_incoming_message_works<Runtime, AllPalletsWithoutSystem, MPI>(
 	collator_session_key: CollatorSessionKeys<Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	sibling_parachain_id: u32,
 	local_relay_chain_id: NetworkId,
@@ -272,6 +273,7 @@ pub fn relayed_incoming_message_works<Runtime, AllPalletsWithoutSystem, MPI>(
 				sibling_parachain_id.into(),
 				included_head,
 				&alice,
+				&slot_durations,
 			);
 
 			// set up relayer details and proofs

cumulus/parachains/runtimes/bridge-hubs/test-utils/src/test_cases/mod.rs

@@ -45,7 +45,7 @@ use frame_system::pallet_prelude::BlockNumberFor;
 use parachains_common::AccountId;
 use parachains_runtimes_test_utils::{
 	mock_open_hrmp_channel, AccountIdOf, BalanceOf, CollatorSessionKeys, ExtBuilder, RuntimeCallOf,
-	XcmReceivedFrom,
+	SlotDurations, XcmReceivedFrom,
 };
 use sp_runtime::{traits::Zero, AccountId32};
 use xcm::{latest::prelude::*, AlwaysLatest};
@@ -419,6 +419,7 @@ pub fn message_dispatch_routing_works<
 	NetworkDistanceAsParentCount,
 >(
 	collator_session_key: CollatorSessionKeys<Runtime>,
+	slot_durations: SlotDurations,
 	runtime_para_id: u32,
 	sibling_parachain_id: u32,
 	unwrap_cumulus_pallet_parachain_system_event: Box<
@@ -529,6 +530,7 @@ pub fn message_dispatch_routing_works<
 			sibling_parachain_id.into(),
 			included_head,
 			&alice,
+			&slot_durations,
 		);
 		let result =
 			<<Runtime as BridgeMessagesConfig<MessagesPalletInstance>>::MessageDispatch>::dispatch(