Evaluator refactoring continues

Moved several aspects of the EvaluatorManager into their own classes:

  * Added EvaluatorControlHandler that relays evaluator control messages to the actually running Evaluator.
  * Added ContextControlHandler that does the same for context control messages.
  * Added EvaluatorStateManager to manage the state of an Evaluator as observed by the Driver

  This allowed EvaluatorContext to no longer require an EvaluatorManager instance.

  Also, this PR renames all handle methods in EvaluatorManager to more expresive names.
working version of runtests.ps1 script; minor fixes in runreef.ps1
set REEF_TEST_YARN in runtests.ps1 script
Merge pull request #747 from Microsoft-CISL/weimer_evaluatorManagerRefactorings

Evaluator refactoring continues

Tests pass on YARN
Merge pull request #748 from Microsoft-CISL/sergiym_runreef

add script to run unit tests on YARN + minor refactorings in powershell scripts
enabled retained eval for bridge approach: initial checkin
Merge remote-tracking branch 'origin' into reef-bridge