add unit test: close evaluator immediately after allocation.
cosmetic changes to teh code
minor refactoring and logging fixes in the Evaluators class
implemented EvaluatorManager.toString()
Merge branch 'master' into sergiym_close_eval

Conflicts:
	reef-common/src/main/java/com/microsoft/reef/runtime/common/driver/evaluator/Evaluators.java
This closes #828. The `synchronzized` block in `EvaluatorManager.close()` was ending a little too early, which created the race condition above.
Merge branch 'master' of github.com:Microsoft-CISL/REEF into sergiym_close_eval
fix the race condition when adding and removing evaluators
Merge branch 'master' of github.com:Microsoft-CISL/REEF into sergiym_close_eval
Merge branch 'sergiym_close_eval' of github.com:Microsoft-CISL/REEF into sergiym_close_eval_fix
add FIXME comments for the temporary fix of the issue #828 and #839
Merge pull request #833 from Microsoft-CISL/sergiym_close_eval_fix

Fix the race condition when adding and removing new evaluators