All of the additional lookup logic was essentially duplicating
a relationship property of 'load_on_pending=True', which tells
SQLAlchemy to lookup the relationship during object creation [1].
So we can dump all of this logic and just use that option.
1. http://docs.sqlalchemy.org/en/latest/orm/relationship_api.html
#sqlalchemy.orm.relationship.params.load_on_pending
Related: blueprint push-notifications
Change-Id: I0e495a50f5cab9b6449825039d7683d77de1e763
6dee4c5942