db.BaseDb: Propose default get_current_version method implementation

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) [2] [3]
• will allow to fix the backends which are missing the appropriate methods (scheduler
• [4], scrubber, vault, see [1]'s description for details...)

Related to swh/infra/sysadm-environment#4228 (closed) Depends on !326 (closed) (revert of [1]) This is the proper fix instead of [1].

• [1] !324 (closed)

• [2] https://forge.softwareheritage.org/source/swh-storage/browse/master/swh/storage/postgresql/storage.py$208-211

• [3] https://forge.softwareheritage.org/source/swh-indexer/browse/master/swh/indexer/storage/init.py$155-157

• [4] https://forge.softwareheritage.org/source/swh-scheduler/browse/master/swh/scheduler/backend.py$76-78

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Migrated from D7949 (view on Phabricator)