[SPARK-6079] Use index to speed up StatusTracker.getJobIdsForGroup()

`StatusTracker.getJobIdsForGroup()` is implemented via a linear scan over a HashMap rather than using an index, which might be an expensive operation if there are many (e.g. thousands) of retained jobs.

This patch adds a new map to `JobProgressListener` in order to speed up these lookups.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #4830 from JoshRosen/statustracker-job-group-indexing and squashes the following commits:

e39c5c7 [Josh Rosen] Address review feedback
6709fb2 [Josh Rosen] Merge remote-tracking branch 'origin/master' into statustracker-job-group-indexing
2c49614 [Josh Rosen] getOrElse
97275a7 [Josh Rosen] Add jobGroup to jobId index to JobProgressListener