improving ZookeeperTaskQueue:
improving ZookeeperTaskQueue:

 * better logging around shutdown
 * moving init_all back to __init__ and making it check for nodes before
   creating, so as not to make unnecessary noise in the zk logs
 * catching signals so we can shutdown gracefully

trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline