feat: Prune states when data is no longer needed

[hallazzang]

Currently some data permanently remain in store(e.g. CurrentEpoch, HistoricalRewards).
By introducing reference counting as in Cosmos SDK, we can safely prune these data.

[dongsam]

@hallazzang At what point do you think it's safe to erase these data?

[hallazzang]

@dongsam Just like Cosmos SDK, we can introduce reference counting and when the reference count becomes zero, we could safely remove the state.

[dongsam]

Up to one HistoricalRewards object is generated per epoch (Day) per StakingCoinDenom. Since the size of HistoricalRewards is also small, it does not seem to occupy an influential data size. So I don't think we have to put it in v1 unless it's a confident and verified code. We could consider putting in on the next release.