[REF-1035] Track ComputedVar dependency per class

[masenf]

• Move computed var tracking to subclass initialization -- no need to recalculate these every time an instance is created (and when using Redis, this can be for every request)
• Move _always_dirty_computed_vars tracking to subclass initialization, again, not needed every time a delta is computed
• Start tracking substates that have always dirty computed vars so they can be recursively recomputed whenever the state changes.
• Use leading underscore for _computed_var_dependencies and _substate_var_dependencies to indicate they are private

Fix #2066

Additional testing is needed, as we should have had some tests that fail without this change.