Use a separate thread for Driver.join() to avoid stalling the executor service.

The unit test changes were made to reproduce AURORA-25 (and happen to be more
true to real-world behavior).

Also, running the scheduler locally prior to this change would silently
reproduce this issue (not joining the server set).

Bugs closed: AURORA-25

Reviewed at https://reviews.apache.org/r/16743/
Client quota checks. Part 1: client side changes.

Reviewed at https://reviews.apache.org/r/16444/