Evaluator state transition events for the local runtime

Closes #938

  * Added `EvaluatorExitTest` that verifies that #938 is indeed an issue. The test still doesn't pass in this PR, but that is because it of #934.
  * Moved `RunnableProcess` to its own package and added observer infrastructure for events from it.
  * Added `ReefRunnableProcessObserver` that sends the events to REEF.