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).
- 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.
The goal here was to hide this class's constructor from users so that we're free to change it later.  I think that making constructors part of public APIs is a bad idea.