Scoru: Introduce machinery to test durable storage against snapshotted

Context

This is the first step towards closing issue #4730 (closed).

This MR doesn't introduce actual tests but it introduces machinery to write such tests easily.

The first commit basically copy-pastes (snapshots) current durable implementation to new library durable_snapshot in tests. The next one brings some machinery to write scenarios.

Next MR: !7727 (merged)

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR