Introducing ContextRepresenters

Closes #953

This PR introduces the new class `ContextRepresenters` which manages all the contexts on a single Evaluator. It is used by `EvaluatorManager` in lieu of the `List` and `Set` it used before. Furthermore, this adds `ContextFactory` which has the sole purpose of creating new `EvaluatorContext` instances for a given Evaluator.
Moved EvaluatorManager.State to be its own class EvaluatorState
Another fix for 763

This fix now keeps `EvaluatorManager` instances around and within them checks whether a received heartbeat shall be processed
Renamed the HelloREEF Job

this is part of #937, but not the full fix.
Results of code review with Shravan
Merge pull request #957 from Microsoft-CISL/weimer_937a

Renamed the HelloREEF Job
Merge pull request #954 from Microsoft-CISL/weimer_953

Introducing ContextRepresenters
Merge branch 'master' of https://github.com/Microsoft-CISL/REEF into weimer_763b
Code review with Shravan
Merge pull request #956 from Microsoft-CISL/weimer_763b

Another fix for 763
Merge remote-tracking branch 'origin' into yingdac-evaluators-checking-back-in

Conflicts:
	reef-common/src/main/java/com/microsoft/reef/runtime/common/driver/evaluator/EvaluatorManager.java
Merge branch 'yingdac-evaluators-checking-back-in' into reef-bridge