cubicweb-container #2532737 parent computation: use container_parent relation only when strictly needed [resolved]

The parent of an entity in a container is computed using an adapter. However the current implementation relies on the systematic use of the container_parent rtype.

As this relation can be quite costly and is in many case not strictly needed we want to:

* have it only for entities that have several upward paths (avoiding high run-time parent computation costs)

* by default, use a simple schema-driven computation

The migration costs associated with the incorporation of container in an existing application have recently been highlighted and this was a sore point.

done in1.0.0
load left0.000
closed by#2c75eb291220 compute container_parent relation only if needed (closes #2532737)