Proxy Protocol support

See #28 for the explanation. This adds a new commit to allow toggling Proxy Protocol via ENV, so we don't need a separate branch.

Interestingly, my observation from last time:

> For some strange reason, it takes about 5 seconds for NGiNX to start responding to Proxy Protocol requests.

...no longer applied on this build. I suspect NGiNX is doing some sort of IP lookup on the source IP reported by Proxy Protocol headers, but I don't know how to verify this, or how to disable the behavior. Suggestions welcome.