This:
- documents what this method is about in the code (only in documentation so far)
- adds a missing test on the cli `swh db version` to check this behaves appropriately
- will allow to clean up duplicated code in existing backends (storage, indexer)
- will allow to fix the backends which are missing the appropriate methods (scrubber,
scheduler, vault, see [1]'s description ...)
Related to T4228
Depends on D7943 (revert of [1])
This is the proper fix instead of [1].
[1] D7907.