Comment 2 for bug 1816109

The following patches work towards this initiative:

Materialized path mixin and field for hierarchical models: https://review.openstack.org/249847
SQLAlchemy column type for materialized path: https://review.openstack.org/251445
Materialized path convenience wrapper: https://review.openstack.org/251455
Use path hybrid property in query filtering: https://review.openstack.org/251513