Cancel all requests in the queue so we can exit.

def cancel_queue(self):
        """
        Cancel all requests in the queue so we can exit.
        """
        q = list(self.queue)
        self.queue = []
        log.debug("Canceling requests: {}".format(q))
        for req in q:
            req.response = APIServerNotRunningErrorResponse()
        for req in q:
            req.signal()