[SPARK-8368] [SPARK-8058] [SQL] HiveContext may override the context class...

[SPARK-8368] [SPARK-8058] [SQL] HiveContext may override the context class loader of the current thread

https://issues.apache.org/jira/browse/SPARK-8368

Also, I add tests according https://issues.apache.org/jira/browse/SPARK-8058.

Author: Yin Huai <yhuai@databricks.com>

Closes #6891 from yhuai/SPARK-8368 and squashes the following commits:

37bb3db [Yin Huai] Update test timeout and comment.
8762eec [Yin Huai] Style.
695cd2d [Yin Huai] Correctly set the class loader in the conf of the state in client wrapper.
b3378fe [Yin Huai] Failed tests.