Kill drivers in postStop() for Worker.
 JIRA SPARK-1068:https://spark-project.atlassian.net/browse/SPARK-1068
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Set ContextWaiter's var stopped to be true if wait() has been triggered
[STREAMING-58]: https://spark-project.atlassian.net/browse/STREAMING-58
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/incubator-spark
Merge branch 'master' of https://github.com/apache/spark