Disallow duplicate VRF keys for pool registration

[teodanciu]

Description

Whenever we register or re-register a pool, starting with version 11, we check that the provided VRF key is not in use by another pool.
When we reap the pools, we also forget their VRF keys, so they become free to be used again.
Resolves #5159

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 an awesome PR. I love all the additions to the test suite.

However, it is missing one critical part. Once we hard fork into protocol version 11, the contents of psVRFKeyHashes will be empty, which will totally screw up the whole logic in this PR.
We need to also adjust the HARDFORK rule to populate that field with data from all of the psStakePools and psFutureStakePools on the hard fork boundary.

[lehins]

This is a beautiful piece of work!
Thank you!