Fix drep delegation invariant preservation

[lehins]

Description

This PR fixes two bugs that where introduced in #4652:

1. Delegating to an SPO by a stake credential that was already delegated to a DRep violated the internal invariant leading to an inconsistency in which upon this DRep unregistration the delegation would not get cleaned up. Thus it would lead to a way to surpass the protection of delegation to a non-registered DRep
2. For testing and benchmarking we have ability to inject DReps and their delegations through the Genesis file. This injection was broken for DReps, since it did not maintain the internal invariant of DRep delegations

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

The refactored version is much easier to grasp, and great bug catch, so subtle!

It took me a while to understand that the old code in Transition was actually doing the delegations, especially I couldn't understand where the oldStakePool <|> newStakePool is handled. But after staring enough at it I can see it in processDelegation.

Anyway! Looks good to me