Refactor Cook scheduler back end (apache#20)

* Refactor Cook scheduler backend

* Fix bug

* Support RetrieveAliveExecutorIds

* Only expose getExecutorIds

* Mv all conf into scheduler context

* Fix style

* More debugging info

* address comments

(cherry picked from commit e8af17b)
(cherry picked from commit 4042429)
(cherry picked from commit 52518f3)
(cherry picked from commit 71da70d)

Author: WenboZhao