Implement circuit extension unification from prop358
We're going to follow the recommendation of https://spec.torproject.org/proposals/358-unified-handshake-extensions.html to prevent circuit handshakes extension from being needlessly duplicated.
We're going to follow the recommendation of https://spec.torproject.org/proposals/358-unified-handshake-extensions.html to prevent circuit handshakes extension from being needlessly duplicated.