[SPARK-9519] [YARN] Confirm stop sc successfully when application was killed

Currently, when we kill application on Yarn, then will call sc.stop() at Yarn application state monitor thread, then in YarnClientSchedulerBackend.stop() will call interrupt this will cause SparkContext not stop fully as we will wait executor to exit.

Author: linweizhong <linweizhong@huawei.com>

Closes #7846 from Sephiroth-Lin/SPARK-9519 and squashes the following commits:

1ae736d [linweizhong] Update comments
2e8e365 [linweizhong] Add comment explaining the code
ad0e23b [linweizhong] Update
243d2c7 [linweizhong] Confirm stop sc successfully when application was killed