feat(state-keeper): Refactor persistence in `StateKeeper`

[slowli]

What ❔

• Splits off miniblock and L1 batch persistence from StateKeeperIO into a separate trait.
• Moves tracking current miniblock / L1 batch number from StateKeeperIO to the state keeper.

Why ❔

• Simplifies further changes to the persistence logic, e.g. toggling saving protective reads.
• Can be used as a building block for generic retrospective L1 batch execution.

Checklist

• [x] PR title corresponds to the body of PR (we generate changelog entries from PRs).
• [x] Tests for the changes have been added / updated.
• [ ] Documentation comments have been added / updated.
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