NetReferenceHelper: add getUniqueId() method that will return a net reference with the special payload bit masked.
ShadowObject: add (package private) getUniqueId() method.
ShadowObject: add (package private) isSpecial() method.
ShadowObject, ShadowClass: move (package private) getClassId() method from ShadowClass to ShadowObject.
ShadowObject: remove the getNetRef() method to (sort of) keep a lid on the raw net reference.
ShadowObject: test equality using getUniqueId() instead of raw net reference, so that objects with the same it but one containing special payload are considered equal.
ShadowObjectTable: only call updateFrom() on an existing object when the other object is "special", i.e., it contains special payload.
ShadowClassTable: avoid using net reference directly, use isSpecial() and getClassId() on the shadow object.