Cleans up any outstanding subscriptions.

async def on_shutdown(self):
        """
        Cleans up any outstanding subscriptions.
        """
        await self._unregister_subscriptions()

        self._accepting = False

        for (ws, _) in self._subscribers:
            await ws.close(code=aiohttp.WSCloseCode.GOING_AWAY,
                           message='Server shutdown')