Update for swh.core 2.0.0

• 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.