They previously had a great deal of near-duplicate code in handling
links and waiting. This collapses all of that into a superclass.

Event should have exactly the same semantics, albeit slightly more clear
lifecycle for events (IMHO).

AsyncResult now behaves more like Event, meaning that:
- the order of notifications is no longer guaranteed
- it is slightly more expensive (one more set ivar), but safer in
concurrent/corner cases.