Make PREV constraint heuristics more sophisticated

[AlexKirkouski]

Now when platform sees that constraint condition is an object deletion, then it wraps message properties into PREV (to avoid getting NULL in message). But prev properties are updated during applying changes, which can be a big overhead. So in theory not entire property should be wrapped (for example PREV(name(stock(s))), but only "dropped" one (for example, name(PREV(stock(s)))).