Feature: Child of Persistent Actor

[Zetanova]

Some PersistentActor types have can contain long persistent events and very short lived events.

An example would be a regular ScheduledJob represented as a PersistentActor.
All events/properties of the ScheduledJob would be long lived and only the ScheduleTimeChanged events short lived
and be deleted.

This behavior can be implemented with a second PersistentActor that tracks only the ScheduleTimes of the ScheduledJob.
It would be great if we could extend the PersistentActor somehow to get some support for controlling and loading such child pid's