Previously stopping the conductor would only prevent it from
accepting new jobs. This change makes it abort the current job
by telling the engine to stop processing tasks after the current
ones finish.
This allows for conductors to gracefully exit when receiving a kill
signal (although the signal handling is not done automatically yet).
Change-Id: Ie6ddcbb2df4508ad1e3f6698c6f4cb2fc26a278f
d1db445461