[change] Change nginx conf to use sockets to redirect traffic to uwsgi and daphne

[nemesifier]

Since the traffic is on the same host, we can use unix sockets to reduce overhead.

For daphne:

upstream channels-backend {
  server unix:{openwisp2_path}/daphne0.sock fail_timeout=0;
}

For uwsgi: https://uwsgi-docs.readthedocs.io/en/latest/Nginx.html#configuring-nginx and I think we'll have to configure the uwsgi.ini to use the unix socket instead of the TCP socket.