This pull request introduces a state.Reader interface for state
accessing.
The interface could be implemented in various ways. It can be pure trie
only reader, or the combination of trie and state snapshot. What's more,
this interface allows us to have more flexibility in the future, e.g.
the
archive reader (for accessing archive state).
Additionally, this pull request removes the following metrics
- `chain/snapshot/account/reads`
- `chain/snapshot/storage/reads`
623b17ba20