Dispute coordinator - Recover disputes on startup

[Lldenaurois]

This PR modifies the dispute coordinator to send a DisputeCoordinatorMessage::Participate for each non-concluded dispute for which the subsystem has no local statement on restarts.

Before entering the main event loop, the subsystem will attempt to resume operation by:

1. Fetching the set of active leaves by issuing ChainSelection::Leaves request.
2. Reconstructing the rolling session window and session infos by calling handle_new_activations for the returned leaves.
3. Loading the non-concluded disputes and their corresponding candidate votes from the database respectively.
4. Sending a DisputeCoordinatorMessage::Participate for any disputes that are missing a local statement.

Along the way the test harness is also refactored to ergonomically support restarting the subsystem, while carrying over any database state between executions.

Fixes #3445

[Lldenaurois]

Additional Question:

Do we need to have any special handling for the dispute availability checks during restart? Or is it safe to ignore dispute availability checks?

[drahnr]

A few nits, generally looks good. The open questions need some more eyes / more dispute knowledgeable eyes.

[Lldenaurois]

@rphmeier How do we keep track of when we've recovered all Statements for all leaves? In this current setting we simply always handle each leaf as it comes in.

[rphmeier]

Suggested change

	let missing_local_statement = validators.iter()
	.enumerate()
	.map(|(index, validator)| (ValidatorIndex(index as _), validator))
	.filter(|(index, validator)|
	voted_indices.contains(index) ||
	state.keystore.key_pair::<ValidatorPair>(validator).map(|v| v.is_none()).unwrap_or_default())
	.count() > 0;
	let missing_local_statement = validators.iter()
	.enumerate()
	.map(|(index, validator)| (ValidatorIndex(index as _), validator))
	.any(|(index, validator)|
	!voted_indices.contains(index) &&
	state.keystore.key_pair::<ValidatorPair>(validator).ok().flatten().map_or(false, |v| v.is_some())
	);

re: https://github.com/paritytech/polkadot/pull/3481/files#r672552299 and https://github.com/paritytech/polkadot/pull/3481/files#r672551425

[rphmeier]

I don't see how this is correct. The iterator contains an entry for each validator which (has voted | has no key). We really want (has not voted & has a key).

map_or is more expressive than using unwrap_or_default implicitly evaluating to false as a default.

[Lldenaurois]

I will write a test to ensure we can ensure this property.

[rphmeier]

missing_local_statement logic seems wrong to me. Other than that it looks fine.

[rphmeier]

bot rebase

[ghost]

Rebasing.

[rphmeier]

@Lldenaurois I triggered the rebase because I wanted to merge, but it seems like you wanted to have another test. Let's block until that's in and then merge