feat: integrate staking pallet into vault-registry and nomination

[gregdhill]

Signed-off-by: Gregory Hill gregorydhill@outlook.com

Integrates the new staking pallet into the vault-registry, nomination pallet and runtime. The main purpose of this is to centralize the stake accounting - many of the fields on the Vault struct have been removed and the Nominator struct is no longer required. Another reason is for reward distribution since rewards should not be accrued on slashed stake.

[nud3l]

To fully review this, I need to understand what the desired outcome of the system is.

Open questions for me are:

Staking pallet

• What does the staking pallet do?
• Is this our way of tracking collateral across vaults and nominators?
• Can we use this with multiple currencies as well?
• What exactly is the stake? Is that just the amount of e.g., DOT deployed by the vault?

Rewards

• What are the implications on the rewards? In our current docs, fees are distributed to vaults 90% based on issued_tokens and 10% based on locked collateral (see https://docs.interlay.io/#/vault/overview?id=pool-based-fee-distribution). It looks like to me rewards are currently only paid based on locked collateral rather than issued_tokens?
• In the future, we will still want to be able to pay rewards on both issued tokens and on collateral locked. In case this is currently not implemented, we need to add this again.

Specification

We need to add a specification for the reward and staking pallet.

Tests

Overall, this changes a lot of logic without adding tests. In the reward crate, a lot of tests are deleted. Please add tests for checking that the assumptions for the reward distributions hold.

[gregdhill]

Staking pallet

• The staking pallet tracks stake / collateral for Vaults and Nominators.
• It currently indexes balances by a currency, but there are efficiency issues with multi-currency staking so I would prefer not to do this.
• The stake here is the collateral for a Vault, but in the rewards pallet this is the unbounded SLA.

Rewards

• This PR has no discernible impact on rewards from what is currently implemented.
• It does however ensure that slashed Vaults do not gain rewards on slashed stake.
• The rewards pallet distributes based on collateral locked + issuance (out-of-scope of this PR).