scripts that edit terse update data try to assert priority
physics simulation tries to assert "volunteer" priority
max priority rules are applied in entity server
* remove EntityItem::_simulationState (only useful to EntitySimulation)
* move code from SimpleEntitySimuation to EntitySimulation
* EntitySimulation now maintans many QSets of entities
* cleanup and clarify use of EnityTree::updateEntity()
* reduced cost of polling for entity expiries
* changed "UpdateFlags" to "DirtyFlags" and clarified what they're for