make in_memory Storage compatible with frozen model entities
since attr based model entities are now frozen in swh.model 0.0.50
Migrated from D2194 (view on Phabricator)
Staging instance, all changes can be removed at any time
since attr based model entities are now frozen in swh.model 0.0.50
Migrated from D2194 (view on Phabricator)