feat: prune abandoned state

[hallazzang]

Description

By introducing reference counting similar to the x/distribution module, we can safely prune abandoned state like no longer used HistoricalRewards, OutstandingRewards, CurrentEpoch, ....

closes: #142

References

See: F1 Fee Distribution Paper's 3. State Requirements for details about state pruning using reference counting.

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[dongsam]

LGTM, Thank you for fixing logic and adding detailed test code. As a result of internal discussions, we think that the PR is not essential for this release, so we will include it in the next release.