Store delegators in pool state

[teodanciu]

Description

Store, in each pool's StakePoolState, the stake credentials that have delegated to that pool.
This set is updated in DELEG rule, for both shelley and conway:

• on (re)delegation
• when the stake credential is unregistered

The set is used to improve the performance of removing delegations when retiring the pool.
Based on PR: #5330

Closes: #4990

Checklist

• Commits in meaningful sequence and with useful messages.
• Tests added or updated when needed.
• CHANGELOG.md files updated for packages with externally visible changes.
  NOTE: New section is never added with the code changes. (See RELEASING.md).
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary.
  NOTE: If bounds change in a cabal file, that package itself must have a version increase. (See RELEASING.md).
• Code formatted (use scripts/fourmolize.sh).
• Cabal files formatted (use scripts/cabal-format.sh).
• CDDL files are up to date (use scripts/gen-cddl.sh)
• hie.yaml updated (use scripts/gen-hie.sh).
• Self-reviewed the diff.

[lehins]

This is a great piece of work. There is only one real problem in this PR and the rest of the comments are things that I've notice that can be improved thanks to your new changes.

[lehins]

This will require processDelegationInternal to accept Maybe (AccountState era), instead of Maybe Delegatee, which should be totally doable

Suggested change

	& certPStateL %~ adjustPState stakePool
	& certPStateL %~ unDelegReDelegStakePool cred accountState (Just stakePool)

[lehins]

Instead of unDelegDRep I propose to add the exact same function as I suggested for stake pools in the Shelley DELEG rule, but for DReps and move it to where VState is defined:

-- Update reverse DRep delegation Set whenever stake credential unregisters or changes its delegation. Whenever new delegation matches the old one this is a noop
unDelegReDelegDRep ::
  Credential 'Staking ->
  AccountState era ->
  -- ^ Account that is loosing its current delegation and/or acquiring a new one
  Maybe DRep ->
  -- ^ Potential new delegation. In case when stake credential unregisters this must be `Nothing`.
  VState era ->
  VState era
unDelegReDelegDRep stakeCred accountState mNewDRep = 
  fromMaybe (vsDRepsL %~ addNewDelegation) $ do
    dRep@(DRepCredential dRepCred) <- accountState ^. dRepDelegationAccountStateL
    pure $ 
      -- There is no need to update set of delegations if delegation is unchanged
      if Just dRep == mNewStakePool
      then id
      else 
        vsDRepsL %~ addNewDelegation . Map.adjust (drepDelegsL %~ Set.delete stakeCred) dRepCred)
  where
    addNewDelegation = 
      case mNewDRep of
        Just (DRepCredential dRepCred) ->
          Map.adjust (drepDelegsL %~ Set.insert stakeCred) dRepCred
        Nothing -> id

[lehins]

No longer needed

Suggested change

	adjustPState newPool =
	(psStakePoolsL %~ Map.adjust (spsDelegsL %~ Set.insert stakeCred) newPool)
	. unDelegStakePool stakeCred mCurDelegatee (Just newPool)

[lehins]

Suggested change

	& certVStateL %~ unDelegDRep stakeCred mCurDelegatee
	& certVStateL %~ unDelegReDelDRep stakeCred mAccountState (Just dRep)

[lehins]

This will need to be slightly rewritten:

Suggested change

	dReps
	| preserveIncorrectDelegation = cState ^. certVStateL . vsDRepsL
	| otherwise = cState' ^. certVStateL . vsDRepsL
	in if preserveIncorrectDelegation
	then cState & certVStateL %~ case dRep of
	DRepCredential dRepCred ->
	vsDRepsL %~ Map.adjust (drepDelegsL %~ Set.insert stakeCred) dRepCred
	_ -> id
	else cState'

And all the stuff below is no longer needed.