Add simple actors to the utils package in core.

This was born of a need for a fast and simple actor libary without pulling
in a heavyweight dependency. The actors support:
* Simple 'tell' pattern
* Simple 'ask' pattern with a Future[Any] result
* Reuse of the Runnable used to start the execution
Partially done rework of HubStage
Remove the actor from the HubStage

The model now requires subclasses of HubStage to manage the read requests from Nodes, and complete them as appropriate.