versioner: use a single work queue.

Previously, each thread was assigned a fixed list of work, and the main
thread would block until every thread was finished, leading to most
cores sitting idle for the last few hundred milliseconds while a few
particularly long running threads would keep working. Use a single work
queue to evenly distribute load across the threads.

Bug: http://b/32748936
Test: python run_tests.py
Change-Id: I80e231ece3b95e2721a32f658905841b89a8dc3b