[Feature] StakeTracker P1 - VotersList

[ruseinov]

Original issue: paritytech/polkadot-sdk#443
Original PR: #13079
Splitting up #12744

polkadot companion: paritytech/polkadot#6527

Terminology

Approval stake - a sum of all the nominator/nomination pools stakes that are backing this validator + self-stake.

Goals

The original goal of this series of PR's is to introduce a list of validators sorted by their approval stake, therefore eliminating the need for Max Validator Intentions parameter. This way we'll be able to look into top x validators in the list to get a set of electable ones.
While doing that we also introduce a concept of EventListeners into Staking, that would allow us to hook into various staking methods acting upon emitted events. In this particular case this allows us to delegate the sorted list maintenance to a 3rd party EventListener.

Technical details

This PR introduces a new Staking-related abstraction called OnStakingUpdate that basically provides the ability to subscribe to certain events that Staking fires and act upon them.

/// A generic staking event listener.
/// Note that the interface is designed in a way that the events are fired post-action, so any
/// pre-action data that is needed needs to be passed to interface methods.
/// The rest of the data can be retrieved by using `StakingInterface`.
pub trait OnStakingUpdate<AccountId: Clone, Balance: Copy> {
	/// Fired when the stake amount of someone updates.
	///
	/// Also called when someone stakes for the first time. (TODO: is it? this is why we need unit
	/// tests for this pallet alone).
	///
	/// This is effectively any changes to the bond amount, such as bonding more funds, and
	/// unbonding.
	fn on_stake_update(who: &AccountId, prev_stake: Option<Stake<AccountId, Balance>>);
	/// Fired when someone sets their intention to nominate, either new, or existing one.
	fn on_nominator_update(who: &AccountId, prev_nominations: Vec<AccountId>);
	/// Fired when someone sets their intention to validate, either new, or existing one.
	fn on_validator_update(who: &AccountId);
	/// Fired when someone removes their intention to validate, either due to chill or nominating.
	fn on_validator_remove(who: &AccountId); // only fire this event when this is an actual Validator
	/// Fired when someone removes their intention to nominate, either due to chill or validating.
	fn on_nominator_remove(who: &AccountId, nominations: Vec<AccountId>); // only fire this if this is an actual Nominator
	/// fired when someone is fully unstaked.
	fn on_unstake(who: &AccountId); // -> basically `kill_stash`
}

The first event listener implementing this interface, also introduced in this PR, is called pallet-stake-tracker which currently takes care of maintaining VoterList (a sorted list of nominators and validator's self-stake), effectively moving this logic out of Staking. It's also supplying Staking with a read-only version of this list, which is currently implemented by adding some defensive checks into insert/updated/remove methods.

Some other things that were introduced in this PR include:

1. Staking tests that verify the correct OnStakingUpdate events are being fired with the right arguments.
2. A read-only wrapper for SortedListProvider, that stubs out update/insert/remove methods and only leaves unsafe methods to allow for easier benchmarking/testing/migrations.
3. CurrencyToVote moved from frame_support to primitives/staking.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-check-benches
Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2780536

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.