Support HAPROXY proxy protocol on the server side

HAProxy defines a TCP-based "proxy protocol" that wraps / prefixes reverse-proxy traffic with metadata from the original client.

See http://www.haproxy.org/download/1.8/doc/proxy-protocol.txt.

**Issue by [prettynatty](https://github.com/prettynatty)**
_Wednesday Jan 20, 2016 at 14:15 GMT_
_Originally opened as https://github.com/akka/akka/issues/19541_

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Akka HTTP lacks proxy protocol support.
There already was a demand in spray/spray#846. It's impossible to implement anti-flood scenarios in case of plain tcp connections when there is no way to determine real client ip via HTTP headers passed from reverse proxy.

Start by reading the HAProxy proxy protocol specification linked in the issue and then trace Akka HTTP's server-side TCP connection handling. The issue names no files or tests; done means accepting the protocol metadata and making the original client IP available for server-side anti-flood handling.

Written by the indexing model from the issue text.