Replace HTTP/HTTPS proxy with SOCKS proxy

[JeremyRand]

Replacing the HTTP/HTTPS proxy with a SOCKS proxy would have the following benefits:

1. Fix HTTP bugs (non-GET requests aren't handled correctly, and the request headers are partially broken right now).
2. Allow clients other than web browsers to use the proxy more easily.
3. Make it easier to implement Tor stream isolation later.

See SOCKS5Server.js for a mostly-working drop-in replacement for the HTTP/HTTPS proxy that we're using now.

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[phelixbtc]

There is a lean SOCKS proxy here: https://github.com/internetarchive/warcprox/blob/master/warcprox/mitmproxy.py