Mitigate guard discovery by pinning middle node

Hello,

inspired by the recent discussions on guard discovery, I went ahead and implemented a small patch for Tor that tries to help defend against Hidden Service guard discovery attacks.

It basically allows the operator to specify a set of nodes that will be pinned as middle nodes in Hidden Service rendezvous circuits. The option only affects HS rendezvous circuits and nothing else.

Of course, it doesn't fix guard discovery, it just pushes guard discovery to the next hop, so that they need to compromise two boxes to win.

You can find my branch in 'sticky_mids' at https://git.torproject.org/user/asn/tor.git .

(Here it is in HTTP shape: https://gitweb.torproject.org/user/asn/tor.git/shortlog/refs/heads/sticky_mids )

[This is the trac version of https://lists.torproject.org/pipermail/tor-dev/2014-November/007730.html]