Better implementation of the reporting emitter thread.
Review Request #3057 - Created Oct. 30, 2015 and submitted
Previously it used twitter.commons.threading.PeriodicThread, which it turns out just calls time.sleep() in a loop. So we were wasting up to half a second on a no-op pants shutdown just for that thread to exit. Now we use proper event signaling, so the thread can be interrrupted immediately. This also gets rid of the t.c.threading dep entirely.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/88238208.
Review request changed