Ensure doc timers are invalidated when document is deleted

Ensure document timer events do not fire if the underlying document has been deleted.

While I understand it is feasible to support the ability for doc timers to fire regardless of deletion, doing so is an unnecessary overlap between docTimer and cronTimer.

User should use doc timers when they wish timers follow document lifecycle. If they wish to follow time, and a document is only a context, it should be done via cronTimer.