Reduce the number of lists/counters associated with a zone #39

Children, callbacks, non-local callbacks and scheduled tasks.
The names are ambiguous and the need to keep so many more or less equivalent lists has a strong code smell.

Reduce the number of lists to one or two, namely:

• (optional) a list callbacks that still need to be called (triggered by activity within the zone completing)
• a list of everything else (sub-zones, tasks and resources - everything that has a .signal() method and will eventually un-register itself from the zone).