Refactor timers

[richardartoul]

This P.Rs refactors ScheduleInvocationRequest into ScheduleSelfTimer such that:

1. Timers can only be scheduled by actors on themselves.
2. Actors run "node local" and will never cause an inter-node invocation. Even more specifically, timers are now specific to the instance/activation that created them and will not run if the actor that created them is no longer activated in-memory and they will not trigger re-activation if they fire after an actor has been deactivated.