- wrap apply_async parameter notification_id in a str() argument

- check if service_callback_api exist before putting tasks on queue
- create_service_callback_api in tests before asserting if send_delivery_status_to_service has been called.

app/dao/notifications_dao.py

@@ -460,7 +460,7 @@ def dao_get_notifications_by_to_field(service_id, search_term, statuses=None):
 
 
 @statsd(namespace="dao")
-def dao_get_notifications_by_reference(references):
+def dao_get_notifications_by_references(references):
     return Notification.query.filter(
         Notification.reference.in_(references)
     ).all()