job_queue: set prefetch count to 1
job_queue: set prefetch count to 1

This prevents the consumer to pull the entire queue into its local
buffer, where the jobs would get stuck for a while.

travis-ci/worker,Acidburn0zzz/worker,henrikhodne-abandoned/worker,henrikhodne-abandoned/worker,henrikhodne-abandoned/worker,travis-ci/worker,Acidburn0zzz/worker,solarce/worker,solarce/worker,travis-ci/worker,Acidburn0zzz/worker,solarce/worker