PROXY protocol v1 / v2 support

[elrik75]

Behind a reverse-proxy or a load balancer, you often need to get the original client IP.
A lot of load balancer provide these information with the PROXY protocol - HAproxy for exemple.
I have a load balancer that provide only this method to get the client info, and I would not be the only one.

Here a doc of the protocol: http://www.haproxy.org/download/2.4/doc/proxy-protocol.txt

In a nutshell, it's one line added on top of the body, here for the v1 protocol:

    PROXY TCP4 192.168.0.1 192.168.0.11 56324 443\r\n
    GET / HTTP/1.1\r\n
    Host: 192.168.0.11\r\n
    \r\n

This protocol is implemented in Gunicorn, and can be activated with --proxy-protocol: https://docs.gunicorn.org/en/stable/settings.html#proxy-protocol

edit: A NodeJS parser: https://github.com/racker/node-proxy-protocol/blob/master/index.js