Refactor Payee and RewardDestination for split and controller removal

[rossbulat]

This is an issue to track refactoring efforts of RewardDestination. The goals are two-fold:

• To remove the Controller variant from RewardDestination enum.
• To introduce a Split variant to compound a percentage of rewards, and send the remaining as free balance, if any, to a specified account.

The proposed solution that this issue will pursue is that of a lazy migration. There are around 69k Payee records in storage now, so a single block migration is not a viable proposition.

A proposed new enum,PayoutDestination, removes Controller, adds Split, and renames the variants to account for the fact that the stash & controller terminology is being deprecated in staking. This new enum will be used in a new Payees storage item.

What has come out of a Polkadot Forum discussion is also support for a simplified verb-based enum, that also uses Split to account for 100% of the payout to go to an account as free balance, without the need for a separate variant to do that.

#[derive(PartialEq, Eq, Copy, Clone, Encode, Decode, RuntimeDebug, TypeInfo, MaxEncodedLen)]
pub enum PayoutDestination<AccountId> {
  /// Pay into the stash account and add to bond.
  Stake,
  /// Pay the specified percentage to the specified account as free balance, and pay the rest, if
  /// any, into the stash account and add to bond. 0% and 100% should be disallowed and handled as
  /// `Stake` and `Free` respectively.
  Split((Perbill, AccountId)),
  /// Pay into an account as free balance.
  Deposit(AccountId),
  /// Receive no payout.
  Forgo,
}

Once all Payee records have been migrated to a new Payees storage item, we can confidently remove the former and replace RewardDestination with PayoutDestination throughout the codebase.

Migrating RewardDestination to PayoutDestination:

• Stash -> Deposit(stash)
• Controller -> Deposit(controller)
• Account(AccountId) -> Deposit(account)
• Staked -> Stake
• None -> Forgo.

PR 1: Introduce new items, , update_payee to migrate to PayoutDestination / Payees.

paritytech/substrate#14451

[do lazy migration via update_payee]

PR 2:

• Remove RewardDestination enum and Payee storage items.
• Remove get_payout_destination and use storage getter.

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/revision-of-rewarddestination-to-account-for-split-and-controller-removal/3360/1