[SPARK-4754] Refactor SparkContext into ExecutorAllocationClient

This is such that the `ExecutorAllocationManager` does not take in the `SparkContext` with all of its dependencies as an argument. This prevents future developers of this class to tie down this class further with the `SparkContext`, which has really become quite a monstrous object.

cc'ing pwendell who originally suggested this, and JoshRosen who may have thoughts about the trait mix-in style of `SparkContext`.

Author: Andrew Or <andrew@databricks.com>

Closes #3614 from andrewor14/dynamic-allocation-sc and squashes the following commits:

187070d [Andrew Or] Merge branch 'master' of github.com:apache/spark into dynamic-allocation-sc
59baf6c [Andrew Or] Merge branch 'master' of github.com:apache/spark into dynamic-allocation-sc
347a348 [Andrew Or] Refactor SparkContext into ExecutorAllocationClient