feat(listeners): support listening on inherited sockets

[lukaslihotzki-f]

Configure the listener like this:

listeners:
  - fd: 3
    #port: 8008
    #tls: false
    #bind_addresses: ["::1"]
    type: http
    x_forwarded: true
    resources:
      - names: [client, federation]
        compress: false

Then, you can use this command:

systemd-socket-activate -l [::]:8008 python -m synapse.app.homeserver --config-path homeserver.yaml

systemd-socket-activate does not require any systemd service running and always starts passing FDs starting with 3. When the first request arrives, synapse will start and respond to it. synapse will keep running and directly handling future requests.