[SPARK-6629] cancelJobGroup() may not work for jobs whose job groups are...

[SPARK-6629] cancelJobGroup() may not work for jobs whose job groups are inherited from parent threads

When a job is submitted with a job group and that job group is inherited from a parent thread, there are multiple bugs that may prevent this job from being cancelable via `SparkContext.cancelJobGroup()`:

- When filtering jobs based on their job group properties, DAGScheduler calls `get()` instead of `getProperty()`, which does not respect inheritance, so it will skip over jobs whose job group properties were inherited.
- `Properties` objects are mutable, but we do not make defensive copies / snapshots, so modifications of the parent thread's job group will cause running jobs' groups to change; this also breaks cancelation.

Both of these issues are easy to fix: use `getProperty()` and perform defensive copying.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #5288 from JoshRosen/localProperties-mutability-race and squashes the following commits:

9e29654 [Josh Rosen] Fix style issue
5d90750 [Josh Rosen] Merge remote-tracking branch 'origin/master' into localProperties-mutability-race
3f7b9e8 [Josh Rosen] Add JIRA reference; move clone into DAGScheduler
707e417 [Josh Rosen] Clone local properties to prevent mutations from breaking job cancellation.
b376114 [Josh Rosen] Fix bug that prevented jobs with inherited job group properties from being cancelled.