Add support HAproxy proxy protocol v2

[Elbandi]

This patch allow enable haproxy proxy protocol (https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt), so backend gets information about connection addresses

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage decreased (-0.3%) to 92.61% when pulling 3bdf0b7 on Elbandi:haproxy-protocol into a312726 on square:master.

[csstaub]

Hi @Elbandi! Thanks so much for your pull request. I wasn't aware of this protocol, but it looks perfect for a proxy like Ghostunnel and I think we should definitely add support for this.

I think this kind of feature probably warrants adding a new integration test. Is there any way that we could set up an integration test running against HAProxy that verifies this implementation is (and stays) correct?

[csstaub]

Thanks for adding a unit test. I will try to get this merged next week.

[csstaub]

Merged in 8f78d6e

[csstaub]

For some reason GitHub isn't picking up the merge, so I'll close this manually. Thank you for the pull request @Elbandi!