docs: def unregister(self, itemtype, item_id):
          '''
          deregister the item in zookeeper /list/
          itemtype must be a Zooborg constant
          item_id must be a string
          '''
          if itemtype not in [ZooConst.CLIENT, ZooConst.WORKER, ZooConst.BROKER]:
              raise Exception('Zooborg.unregister: invalid type')
          self.initconn()
          self.zk.ensure_path("/distark/" + itemtype + "/list")
          path=''.join(['/distark/' + itemtype + '/list/', item_id])
          if self.zk.exists(path):
              self.zk.delete(path, recursive=True)