This method compares the version stored in the database in the dbversion
table with the currently declared version.
This current version is declared as a simple current_version attribute
on the Db class. It must be updated jointly in 30-swh-schema.sql when
modifying the db schema. Note that if this is forgotten, the added test
test_dbversion should fail.
Related to T2525.
Also:
- Code cleanup: fix import statements swh.storage.common content has been moved to swh.core.db.common a while ago.
- Fix pytest_plugin's database janitor: do not truncate the dbversion table