Previously, Scene.Inventory.DeRezObjects() forced the persistence of prims before deletion.
This is necessary so that freshly delinked prims can be deleted (otherwise they remain as parts of their old group and reappear on server restart).
However, DeRezObjects() deleted to user inventory, which is not required by llDie() or direct region module unlink and deletion.
Therefore, forced persistence has been pushed down into Scene.UnlinkSceneObject() to be more general, this is still on the DeRezObjects() path.
Uncommented TestDelinkPersistence() since this now passes.
Tests required considerable elaboration of MockRegionDataPlugin to reflect underlying storing of parts.
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