Generate redis key for event at timestamp.

        :param event: event name
        :param ts: timestamp, default to current timestamp if left as None

def _keygen(self, event, ts=None):
        """Generate redis key for event at timestamp.

        :param event: event name
        :param ts: timestamp, default to current timestamp if left as None
        """
        return "%s:%s" % (self.namespace(ts or time.time()), event)