message: Run _TPUPollingThread as a daemon thread
diff: @@ -60,6 +60,7 @@ class _TPUPollingThread(threading.Thread):
  def __init__(self, cluster, session):
  super(_TPUPollingThread, self).__init__()
  + self.daemon = True
  self._running = True
  self._session_closed = False
  self._cluster = cluster
  