Multi-Block-Migrations, `poll` hook and new System callbacks (#1781)

This MR is the merge of
https://github.com/paritytech/substrate/pull/14414 and
https://github.com/paritytech/substrate/pull/14275. It implements
[RFC#13](https://github.com/polkadot-fellows/RFCs/pull/13), closes
https://github.com/paritytech/polkadot-sdk/issues/198.

----- 

This Merge request introduces three major topicals:

1. Multi-Block-Migrations
1. New pallet `poll` hook for periodic service work
1. Replacement hooks for `on_initialize` and `on_finalize` in cases
where `poll` cannot be used

and some more general changes to FRAME.  
The changes for each topical span over multiple crates. They are listed
in topical order below.

# 1.) Multi-Block-Migrations

Multi-Block-Migrations are facilitated by creating `pallet_migrations`
and configuring `System::Config::MultiBlockMigrator` to point to it.
Executive picks this up and triggers one step of the migrations pallet
per block.
The chain is in lockdown mode for as long as an MBM is ongoing.
Executive does this by polling `MultiBlockMigrator::ongoing` and not
allowing any transaction in a block, if true.

A MBM is defined through trait `SteppedMigration`. A condensed version
looks like this:
```rust
/// A migration that can proceed in multiple steps.
pub trait SteppedMigration {
	type Cursor: FullCodec + MaxEncodedLen;
	type Identifier: FullCodec + MaxEncodedLen;

	fn id() -> Self::Identifier;

	fn max_steps() -> Option<u32>;

	fn step(
		cursor: Option<Self::Cursor>,
		meter: &mut WeightMeter,
	) -> Result<Option<Self::Cursor>, SteppedMigrationError>;
}
```

`pallet_migrations` can be configured with an aggregated tuple of these
migrations. It then starts to migrate them one-by-one on the next
runtime upgrade.
Two things are important here:
- 1. Doing another runtime upgrade while MBMs are ongoing is not a good
idea and can lead to messed up state.
- 2. **Pallet Migrations MUST BE CONFIGURED IN `System::Config`,
otherwise it is not used.**

The pallet supports an `UpgradeStatusHandler` that can be used to notify
external logic of upgrade start/finish (for example to pause XCM
dispatch).

Error recovery is very limited in the case that a migration errors or
times out (exceeds its `max_steps`). Currently the runtime dev can
decide in `FailedMigrationHandler::failed` how to handle this. One
follow-up would be to pair this with the `SafeMode` pallet and enact
safe mode when an upgrade fails, to allow governance to rescue the
chain. This is currently not possible, since governance is not
`Mandatory`.

## Runtime API

- `Core`: `initialize_block` now returns `ExtrinsicInclusionMode` to
inform the Block Author whether they can push transactions.

### Integration

Add it to your runtime implementation of `Core` and `BlockBuilder`:
```patch
diff --git a/runtime/src/lib.rs b/runtime/src/lib.rs
@@ impl_runtime_apis! {
	impl sp_block_builder::Core<Block> for Runtime {
-		fn initialize_block(header: &<Block as BlockT>::Header) {
+		fn initialize_block(header: &<Block as BlockT>::Header) -> RuntimeExecutiveMode {
			Executive::initialize_block(header)
		}

		...
	}
```

# 2.) `poll` hook

A new pallet hook is introduced: `poll`. `Poll` is intended to replace
mostly all usage of `on_initialize`.
The reason for this is that any code that can be called from
`on_initialize` cannot be migrated through an MBM. Currently there is no
way to statically check this; the implication is to use `on_initialize`
as rarely as possible.
Failing to do so can result in broken storage invariants.

The implementation of the poll hook depends on the `Runtime API` changes
that are explained above.

# 3.) Hard-Deadline callbacks

Three new callbacks are introduced and configured on `System::Config`:
`PreInherents`, `PostInherents` and `PostTransactions`.
These hooks are meant as replacement for `on_initialize` and
`on_finalize` in cases where the code that runs cannot be moved to
`poll`.
The reason for this is to make the usage of HD-code (hard deadline) more
explicit - again to prevent broken invariants by MBMs.

# 4.) FRAME (general changes)

## `frame_system` pallet

A new memorize storage item `InherentsApplied` is added. It is used by
executive to track whether inherents have already been applied.
Executive and can then execute the MBMs directly between inherents and
transactions.

The `Config` gets five new items:
- `SingleBlockMigrations` this is the new way of configuring migrations
that run in a single block. Previously they were defined as last generic
argument of `Executive`. This shift is brings all central configuration
about migrations closer into view of the developer (migrations that are
configured in `Executive` will still work for now but is deprecated).
- `MultiBlockMigrator` this can be configured to an engine that drives
MBMs. One example would be the `pallet_migrations`. Note that this is
only the engine; the exact MBMs are injected into the engine.
- `PreInherents` a callback that executes after `on_initialize` but
before inherents.
- `PostInherents` a callback that executes after all inherents ran
(including MBMs and `poll`).
- `PostTransactions` in symmetry to `PreInherents`, this one is called
before `on_finalize` but after all transactions.

A sane default is to set all of these to `()`. Example diff suitable for
any chain:
```patch
@@ impl frame_system::Config for Test {
 	type MaxConsumers = ConstU32<16>;
+	type SingleBlockMigrations = ();
+	type MultiBlockMigrator = ();
+	type PreInherents = ();
+	type PostInherents = ();
+	type PostTransactions = ();
 }
```

An overview of how the block execution now looks like is here. The same
graph is also in the rust doc.

<details><summary>Block Execution Flow</summary>
<p>

![Screenshot 2023-12-04 at 19 11
29](https://github.com/paritytech/polkadot-sdk/assets/10380170/e88a80c4-ef11-4faa-8df5-8b33a724c054)

</p>
</details> 

## Inherent Order

Moved to https://github.com/paritytech/polkadot-sdk/pull/2154



---------------


## TODO

- [ ] Check that `try-runtime` still works
- [ ] Ensure backwards compatibility with old Runtime APIs
- [x] Consume weight correctly
- [x] Cleanup

---------

Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Liam Aharon <[email protected]>
Co-authored-by: Juan Girini <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: Gavin Wood <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>