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
fix the deadlock from issue #840
Merge pull request #841 from Microsoft-CISL/sergiym_evaluators_deadlock_fix

fix the deadlock from issue #840
reenable the unit test that throws exception from TaskStop handler.
Refactor the FailTaskTest code and around
return null from FailTaskStop.call()
add a new unit test for the task that throws from teh CloseEvent handler
Merge pull request #843 from Microsoft-CISL/sergiym_fail_task_stop

Re-enable unit test that throws exception from TaskStop handler
Merge pull request #844 from Microsoft-CISL/sergiym_fail_task_close

unit test for the task that throws from CloseEvent handler
minor logging improvemtns in the pool benchmark task