Epochs: Introduce the last-epoch validator set

[insumity]

Problem

Currently, we cannot move to an epoch-based approach as is because key-assignment replacements work on a per-block basis. To tackle this, we intend to store the last validator set that was sent to a consumer chain at the last epoch. This way, we could easily create ValidatorUpdates to send down to the consumer chains.

Closing criteria

1. Introduce the relevant state and getter/setters
2. (optional) Move soft-opt-out to only set the 95% top validators. The implication of this would be that the validators that opted out would still get rewards but the consumer chain would be faster because we won't have rounds with non-existent proposers.
3. Add a test that would check that we only send VSCPackets once every BlocksPerEpoch (see Epochs: Introduce governance param #1649).