Move swh.storage.db.BaseDb class in swh.core

Since it is meant to be the base to use class to wrap any db access.

This imply:

• extract the code from swh.storage in swh.core (swh/devel/swh-core!305 (closed))
• rewrite scheduler's swh.scheduler.backend.SchedulerBackend class (swh/devel/swh-scheduler#1494 (closed))
• refactor swh.storage.db.BaseDb (swh/devel/swh-storage!901 (closed))
• swh.vault.backend (swh/devel/swh-vault!37 (closed))
• swh.indexer.storage.db

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Migrated from T1495 (view on Phabricator)