Start workers before adding jobs into the queue.
Start workers before adding jobs into the queue.

Allows workers to start early and not have to wait for all jobs to be
added. Queue also seems to have significant overhead when very full, so
this allows running a lot more total jobs.

greggomann/gtest-parallel,google/gtest-parallel,kwiberg/gtest-parallel,ehlemur/gtest-parallel,bbannier/gtest-parallel,pbos/gtest-parallel