[SPARK-7237] Many user provided closures are not actually cleaned

Note: ~140 lines are tests.

In a nutshell, we never cleaned closures the user provided through the following operations:
- sortBy
- keyBy
- mapPartitions
- mapPartitionsWithIndex
- aggregateByKey
- foldByKey
- foreachAsync
- one of the aliases for runJob
- runApproximateJob

For more details on a reproduction and why they were not cleaned, please see [SPARK-7237](https://issues.apache.org/jira/browse/SPARK-7237).

Author: Andrew Or <andrew@databricks.com>

Closes #5787 from andrewor14/clean-more and squashes the following commits:

2f1f476 [Andrew Or] Merge branch 'master' of github.com:apache/spark into clean-more
7265865 [Andrew Or] Merge branch 'master' of github.com:apache/spark into clean-more
df3caa3 [Andrew Or] Address comments
7a3cc80 [Andrew Or] Merge branch 'master' of github.com:apache/spark into clean-more
6498f44 [Andrew Or] Add missing test for groupBy
e83699e [Andrew Or] Clean one more
8ac3074 [Andrew Or] Prevent NPE in tests when CC is used outside of an app
9ac5f9b [Andrew Or] Clean closures that are not currently cleaned
19e33b4 [Andrew Or] Add tests for all public RDD APIs that take in closures