Fix `TrieStateStore.PruneStates()` to work together with fragmented states

[dahlia]

Since fragmented states were introduced:

• https://github.com/planetarium/libplanet/pull/1636
  • https://github.com/planetarium/bencodex.net/pull/50
  • https://github.com/planetarium/bencodex.net/pull/52
  • https://github.com/planetarium/bencodex.net/pull/53
  • https://github.com/planetarium/bencodex.net/pull/55
  • https://github.com/planetarium/bencodex.net/pull/56

TrieStateStore.PruneStates() method has not worked well. Note that there's even a comment addressing this:

https://github.com/planetarium/libplanet/blob/bf2a321af65cd70dda9243e6a8e888a217223928/Libplanet/Store/TrieStateStore.cs#L41-L43

There are two problems:

• [x] It throws ArgumentOutOfRangeException because now IStateStore has keys other than SHA-256 digest.
  • https://github.com/planetarium/NineChronicles.Snapshot/issues/78
• [ ] Reference tracking works only at MPT-level. References between fragmented states also should be tracked.

The former one should be published as a hotfix. The latter one would accompany API changes, so it should be published in the next minor release.

[stale[bot]]

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