Validator set persistence

[pk910]

This PR adds a validators table to the db and persists the finalized validator set there.

The validator cache is responsible for writing and updating validator entries.
It uses a batched approach for mass updates to keep the db load manageable.

This PR touches a lot of code as the validator set accessors changed.
Instead of fetching the full validator set and doing filtering/aggregation logic in the page handlers, the page handlers will now only fetch a filtered subset of the validator set.

This PR should significantly lower the memory consumption of dora when running in a network with large validator sets (holesky, mainnet)

[pk910]

This branch is now running on https://dora-holesky.pk910.de/ for the holesky testnet.
The first results look very promising - dora runs with a total memory consumption of ~2-3GB :)
Compared to a consumption of 10GB before the update, that's quite an improvement.

Issues found:

• Active Validators, Staked Ether & Average Balance stats on index page show 0 during the first few slots of a new epoch (latest epoch stats are still loading)
• Validators list status filter does not work