[SPARK-2321] Several progress API improvements / refactorings

This PR refactors / extends the status API introduced in #2696.

- Change StatusAPI from a mixin trait to a class.  Before, the new status API methods were directly accessible through SparkContext, whereas now they're accessed through a `sc.statusAPI` field.  As long as we were going to add these methods directly to SparkContext, the mixin trait seemed like a good idea, but this might be simpler to reason about and may avoid pitfalls that I've run into while attempting to refactor other parts of SparkContext to use mixins (see #3071, for example).
- Change the name from SparkStatusAPI to SparkStatusTracker.
- Make `getJobIdsForGroup(null)` return ids for jobs that aren't associated with any job group.
- Add `getActiveStageIds()` and `getActiveJobIds()` methods that return the ids of whatever's currently active in this SparkContext.  This should simplify davies's progress bar code.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3197 from JoshRosen/progress-api-improvements and squashes the following commits:

30b0afa [Josh Rosen] Rename SparkStatusAPI to SparkStatusTracker.
d1b08d8 [Josh Rosen] Add missing newlines
2cc7353 [Josh Rosen] Add missing file.
d5eab1f [Josh Rosen] Add getActive[Stage|Job]Ids() methods.
a227984 [Josh Rosen] getJobIdsForGroup(null) should return jobs for default group
c47e294 [Josh Rosen] Remove StatusAPI mixin trait.