Handle an incoming client event.

def _handle_event(self, sid, namespace, id, data):
        """Handle an incoming client event."""
        namespace = namespace or '/'
        self.logger.info('received event "%s" from %s [%s]', data[0], sid,
                         namespace)
        if self.async_handlers:
            self.start_background_task(self._handle_event_internal, self, sid,
                                       data, namespace, id)
        else:
            self._handle_event_internal(self, sid, data, namespace, id)