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 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.
Added a MockRegionDataPlugin to do in-memory persistence for tests since adding this to OpenSim.Data.Null.NullDataStore doesn't seem appropriate
NullDataStore can do nothing because OpenSim only ever retrieve region objects from the database on startup. Adding an in-memory store here would be unecessary overhead.
