split delete task up into per service

we really don't gain anything by running each service delete in sequence
- we get the services, and then just loop through them deleting per
service. By deleting per service in separate tasks, we can take
advantage of parallelism. the only thing we lose is some log lines but I
don't think we're that interested in them.

only set query limit at the move_notifications dao function - the task
doesn't really care about the technical implementation of how it deletes
the notifications

app/dao/service_data_retention_dao.py

@@ -50,3 +50,7 @@ def update_service_data_retention(service_data_retention_id, service_id, days_of
         }
     )
     return updated_count
+
+
+def fetch_service_data_retention_for_all_services_by_notification_type(notification_type):
+    return ServiceDataRetention.query.filter(ServiceDataRetention.notification_type == notification_type).all()