Refactor WhitelistedHashesRepository

[quantumagi]

This repository is fairly loose wrt tracking its own "tip" - i.e. the information in the repository is ill-defined as to how up-to-date it is wrt which tip. It is not capable of rewind on start-up despite scenarios where the other components may be rewinding.

After the overhaul we re-build, the now in-memory repo, from a potentially rewound polls repository on startup.

Some feature we now have with the new repository:

• a detailed and exact history of when hashes were active due to qualifying existence checks with height parameters.
• the ability to register hashes (with their activation height) even before they become active which will assist in future performance improvements (e.g. PR [IBD/Rewind] Optimize VotingManager for batched operations #933).

[quantumagi]

WIP

[quantumagi]

Should be ready for review now