[SPARK-7996] Deprecate the developer api SparkEnv.actorSystem

Changed ```SparkEnv.actorSystem``` to be a function such that we can use the deprecated flag with it and added a deprecated message.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #6731 from ilganeli/SPARK-7996 and squashes the following commits:

be43817 [Ilya Ganelin] Restored to val
9ed89e7 [Ilya Ganelin] Added a version info for deprecation
9610b08 [Ilya Ganelin] Converted actorSystem to function and added deprecated flag