Related T700

This also has the benefit to hide some celery name (which is an
implementation detail from swh.scheduler).

To make sure corruptions such as T680 don't happen again, use the same
normalization function as swh.model before inserting timestamps into our
database.

This makes swh.storage reject non-integer timestamp values as well.

Update tests to reflect this change.

Closes T646

Related T646

Related T646

Related T613

The check_config method allows a dynamic check of the configuration for
a running storage. We can make sure that we have proper permissions on
the object storage as well as the database before running things.