Refactor logic related to Evaluator state

We need to refactor the logic related to EvaluatorState and State enums. It is currently spread across several classes, and is not always consistent. Method names are clumsy, e.g. .isDoneOrFailedOrKilled(); EvaluatorStatusManager class has all its methods synchronized, although having an atomic enum reference would be enough. We also need to add more javadocs and clean up the code for readability.