- Add expected entry points for swh.core 2 db handling new features:

  - add a ``swh.storge.get_datastore()`` function
  - add ``swh.storage.postgreql.storage.Storage.get_current_version()`` method
  - move sql migration scripts in ``swh/storage/sql/upgrades``
  - modify sql initialization scripts to match swh.core 2 (remove
    dbversion management code).

- Update tests to use the new template-based database handling; this
  should have only minimal impact on test execution performances.