Runs each function from ``self.before_first_request_funcs`` once and only once.

def _try_trigger_before_first_request_funcs(self):  # pylint: disable=C0103
        """Runs each function from ``self.before_first_request_funcs`` once and only once."""
        if self._after_first_request_handled:
            return
        else:
            with self._before_first_request_lock:
                if self._after_first_request_handled:
                    return
                for func in self._before_first_request_funcs:
                    func()
                self._after_first_request_handled = True