netty4 only changes (w/o MLSS stuff)
minor refactoring in groupcomm operators
Merge branch 'master' of https://github.com/Microsoft-CISL/REEF into weimer_evaluator_exit
Reduced the logging in `RetainedEvalTest`
Made `EvaluatorContext.close()` idempotent. Added a warning for when that is used.
Added a handler for the client side close request to the retained evaluator example
Commented out RetainedEvalTest as it sometimes failed. Given that all other tests of similar functionality pass, I suspect it is an issue with the test. Also, see #928 for an open issue to clean up that part of the codebase.