Normalize the scheduler db for better `swh db` compatibility

In the next version of swh.core, we expect all the swh.core.db based swh backends to register themselves in the swh.<package>.classes entry point so the swh db tools can easily handle the whole lifecycle of the backend's db easily (especially in integration tests like the docker env or the mirror deployment project).

So be it.

Will need a dependency bump on swh.core before merging.