It's not necessary to expose the internal states in our schema.
An enum type wll be used instead now which contains all internal states.

Advantages:
* This makes it easy to set up handlers in the new StateMachine and
  we can skip reflection code in the look up of a handler for an internal
  state in the StateMachine.
* The use of enum easily enables a priority queue so that jobs who are
  in a more advanced state are processed first.