cubicweb #1625291 consider setting owned_by in an operation and not a hook [open]

In the current implementation, owned_by is set by SetOwnershipHook, one entity after another, which prevents us from using the optimised set relations methods. When doing massive imports there is a non negligible cost here.

However, if we do things that way, there is certainly a reason. Find out. See what can be done.

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