It's a follow-up of #2949. It enables the lookahead collator to
dynamically adjust the aura slot size, which may change during the
runtime upgrade. It also addressed a couple of issues with time
constants I missed in the original PR.

Good news: it works. The parachain successfully switches from sync
backing with 12s slots to async backing with 6s slots.

Bad news: during the transitional period of a single block in which the
actual runtime upgrade is performed, it still gets the old slot duration
of 12s (as it gets it from the best block), resulting in a runtime panic
(logs follow). That doesn't affect the following block production of the
parachain. Ideas on how to improve the situation are appreciated.

<details>

```
2024-02-05 12:59:36.373  INFO tokio-runtime-worker sc_basic_authorship::basic_authorship: [Parachain] 🙌 Starting consensus session on top of parent 0x6fd2d8f904f12c22531bfabf77b16dc84a6a29e45d9ae358aa6547fbf3f0438b    
2024-02-05 12:59:36.373 ERROR tokio-runtime-worker runtime: [Parachain] panicked at /home/s0me0ne/wrk/parity/polkadot-sdk/cumulus/pallets/aura-ext/src/consensus_hook.rs:69:9:
assertion `left == right` failed: slot number mismatch
  left: Slot(142261198)
 right: Slot(284522396)    
2024-02-05 12:59:36.373  WARN tokio-runtime-worker sp_state_machine::overlayed_changes::changeset: [Parachain] 1 storage transactions are left open by the runtime. Those will be rolled back.    
2024-02-05 12:59:36.373  WARN tokio-runtime-worker sp_state_machine::overlayed_changes::changeset: [Parachain] 1 storage transactions are left open by the runtime. Those will be rolled back.    
2024-02-05 12:59:36.373  WARN tokio-runtime-worker basic-authorship: [Parachain] ❗ Inherent extrinsic returned unexpected error: Error at calling runtime api: Execution failed: Execution aborted due to trap: wasm trap: wasm `unreachable` instruction executed
WASM backtrace:
error while executing at wasm backtrace:
    0: 0x4e4a3b - <unknown>!rust_begin_unwind
    1: 0x46cf57 - <unknown>!core::panicking::panic_fmt::h3c280dba88683724
    2: 0x46d238 - <unknown>!core::panicking::assert_failed_inner::hebac5970933beb4d
    3: 0x3d00fc - <unknown>!core::panicking::assert_failed::h640a47e2fb5dfb4b
    4: 0xd0db3 - <unknown>!frame_support::storage::transactional::with_transaction::hcbc31515f81b2ee1
    5: 0x34d654 - <unknown>!<cumulus_pallet_parachain_system::pallet::Call<T> as frame_support::traits::dispatch::UnfilteredDispatchable>::dispatch_bypass_filter::{{closure}}::hb7c2c9a11fa88301
    6: 0x3547db - <unknown>!environmental::local_key::LocalKey<T>::with::h783f2605ae27d6d3
    7: 0x7f454 - <unknown>!<asset_hub_rococo_runtime::RuntimeCall as frame_support::traits::dispatch::UnfilteredDispatchable>::dispatch_bypass_filter::h5e11a01ab97c06c7
    8: 0x7f237 - <unknown>!<asset_hub_rococo_runtime::RuntimeCall as sp_runtime::traits::Dispatchable>::dispatch::h7f8ae4a8fede71ca
    9: 0x26a0f3 - <unknown>!frame_executive::Executive<System,Block,Context,UnsignedValidator,AllPalletsWithSystem,COnRuntimeUpgrade>::apply_extrinsic::h75e524ff34738391
   10: 0x282211 - <unknown>!BlockBuilder_apply_extrinsic. Dropping.    
2024-02-05 12:59:36.374 ERROR tokio-runtime-worker runtime: [Parachain] panicked at /home/s0me0ne/wrk/parity/polkadot-sdk/substrate/frame/aura/src/lib.rs:416:9:
assertion `left == right` failed: Timestamp slot must match `CurrentSlot`
  left: Slot(142261198)
 right: Slot(284522396)    
2024-02-05 12:59:36.374  WARN tokio-runtime-worker sp_state_machine::overlayed_changes::changeset: [Parachain] 1 storage transactions are left open by the runtime. Those will be rolled back.    
2024-02-05 12:59:36.374  WARN tokio-runtime-worker sp_state_machine::overlayed_changes::changeset: [Parachain] 1 storage transactions are left open by the runtime. Those will be rolled back.    
2024-02-05 12:59:36.374  WARN tokio-runtime-worker basic-authorship: [Parachain] ❗

 Inherent extrinsic returned unexpected error: Error at calling runtime api: Execution failed: Execution aborted due to trap: wasm trap: wasm `unreachable` instruction executed
WASM backtrace:
error while executing at wasm backtrace:
    0: 0x4e4a3b - <unknown>!rust_begin_unwind
    1: 0x46cf57 - <unknown>!core::panicking::panic_fmt::h3c280dba88683724
    2: 0x46d238 - <unknown>!core::panicking::assert_failed_inner::hebac5970933beb4d
    3: 0x3d00fc - <unknown>!core::panicking::assert_failed::h640a47e2fb5dfb4b
    4: 0x9ece6 - <unknown>!frame_support::storage::transactional::with_transaction::h26f75cb9f9462088
    5: 0x356d7e - <unknown>!environmental::local_key::LocalKey<T>::with::hbcf2d4e17b48fdb5
    6: 0x7f507 - <unknown>!<asset_hub_rococo_runtime::RuntimeCall as frame_support::traits::dispatch::UnfilteredDispatchable>::dispatch_bypass_filter::h5e11a01ab97c06c7
    7: 0x7f237 - <unknown>!<asset_hub_rococo_runtime::RuntimeCall as sp_runtime::traits::Dispatchable>::dispatch::h7f8ae4a8fede71ca
    8: 0x26a0f3 - <unknown>!frame_executive::Executive<System,Block,Context,UnsignedValidator,AllPalletsWithSystem,COnRuntimeUpgrade>::apply_extrinsic::h75e524ff34738391
    9: 0x282211 - <unknown>!BlockBuilder_apply_extrinsic. Dropping.    
2024-02-05 12:59:36.374 DEBUG tokio-runtime-worker runtime::xcmp-queue-migration: [Parachain] Lazy migration finished: item gone    
2024-02-05 12:59:36.374 ERROR tokio-runtime-worker runtime: [Parachain] panicked at /home/s0me0ne/wrk/parity/polkadot-sdk/cumulus/pallets/parachain-system/src/lib.rs:265:18:
set_validation_data inherent needs to be present in every block!    
2024-02-05 12:59:36.374 ERROR tokio-runtime-worker aura::cumulus: [Parachain] err=Error { inner: Proposing

Caused by:
    0: Error at calling runtime api: Execution failed: Execution aborted due to trap: wasm trap: wasm `unreachable` instruction executed
       WASM backtrace:
       error while executing at wasm backtrace:
           0: 0x4e4a3b - <unknown>!rust_begin_unwind
           1: 0x46cf57 - <unknown>!core::panicking::panic_fmt::h3c280dba88683724
           2: 0x46da8b - <unknown>!core::option::expect_failed::hdf18d99c3adabca7
           3: 0x2134cb - <unknown>!<cumulus_pallet_parachain_system::pallet::Pallet<T> as frame_support::traits::hooks::OnFinalize<<<<T as frame_system::pallet::Config>::Block as sp_runtime::traits::HeaderProvider>::HeaderT as sp_runtime::traits::Header>::Number>>::on_finalize::hf98aac39802896ba
           4: 0x26a9d6 - <unknown>!frame_executive::Executive<System,Block,Context,UnsignedValidator,AllPalletsWithSystem,COnRuntimeUpgrade>::idle_and_finalize_hook::h32775c0df0749d92
           5: 0x26ad9f - <unknown>!frame_executive::Executive<System,Block,Context,UnsignedValidator,AllPalletsWithSystem,COnRuntimeUpgrade>::finalize_block::h15e5a1a6b9ca8032
           6: 0x2822bd - <unknown>!BlockBuilder_finalize_block
    1: Execution failed: Execution aborted due to trap: wasm trap: wasm `unreachable` instruction executed
       WASM backtrace:
       error while executing at wasm backtrace:
           0: 0x4e4a3b - <unknown>!rust_begin_unwind
           1: 0x46cf57 - <unknown>!core::panicking::panic_fmt::h3c280dba88683724
           2: 0x46da8b - <unknown>!core::option::expect_failed::hdf18d99c3adabca7
           3: 0x2134cb - <unknown>!<cumulus_pallet_parachain_system::pallet::Pallet<T> as frame_support::traits::hooks::OnFinalize<<<<T as frame_system::pallet::Config>::Block as sp_runtime::traits::HeaderProvider>::HeaderT as sp_runtime::traits::Header>::Number>>::on_finalize::hf98aac39802896ba
           4: 0x26a9d6 - <unknown>!frame_executive::Executive<System,Block,Context,UnsignedValidator,AllPalletsWithSystem,COnRuntimeUpgrade>::idle_and_finalize_hook::h32775c0df0749d92
           5: 0x26ad9f - <unknown>!frame_executive::Executive<System,Block,Context,UnsignedValidator,AllPalletsWithSystem,COnRuntimeUpgrade>::finalize_block::h15e5a1a6b9ca8032
           6: 0x2822bd - <unknown>!BlockBuilder_finalize_block }
```

</details>

---------

Co-authored-by: Bastian Köcher <[email protected]>