docs: private void shutdown(){
      parent.workerThreads.get(plan).remove(this);
      if (zk != null) {
        try {
          logger.debug("closing " + zk.getSessionId());
          zk.close();
          zk = null;
        } catch (InterruptedException e1) {
          logger.debug(e1);
        }
        logger.debug("shutdown complete");
      }
    }