Listener can be called after destroy

[TarVK]

When calling .destroy() on an Observer that has a debounce delay, the listener may still be called after this delay.
This may be desirable in some cases, but at least is not in all cases. We could either always cancel scheduled updates, or add a flag to either cancel them or not.

Below is a link to the relevant function:
https://github.com/TarVK/model-react/blob/master/src/model/dataHooks/Observer.ts#L108