Add multiple executors

Refactoring / clean up
Adding executors instead of framework messages
Sure, let's discuss this in more depth for a subsequent review.  I understand the concept you mention, but I think that for data nodes or journal nodes, we will want custom functionality, i.e. a heart beat to indicate that it's still alive and the ability to kill and relaunch.  We also might need custom error logging.  For this, a custom executor will benefit us rather than adding too much logic in the scheduler.  Perhaps there is a better design.