1. Removes state variables:
- totalWeightedStakeThisEpoch
- totalFeesCollectedThisEpoch
- numActivePoolsThisEpoch
2. No longer indexes by epoch % 2
3. Renamed event StakingPoolActivated → StakingPoolEarnedRewards.
4. Renamed structs:
- ActivePool → PoolStats. This holds stats for a pool that earned rewards.
- UnfinalizedState → AggregatedStats. This aggregates stats from the former struct.