Replace Daphne by Gunicorn/Uvicorn

We're using daphne as our asgi/wsgi server in production. However, Daphne is still quite young and not as feature complete. Especially, it doesn't support running multiple worker processes, and reloading code without dropping requests.

On the other side, Uvicorn supports ASGI, offers a Gunicorn plugin, meaning we can benefit from the maturity of this WSGI server.