Defang TLS UKS attacks in tor-spec.txt

Our implementations disable session resumption, and don't send
AUTHENTICATE before checking CERTS.  It turns out that these are
pretty important: see discussion of "triple handshake" and related
UKS-enabled attacks at

https://lists.torproject.org/pipermail/tor-dev/2014-March/006372.html

Basically, session resumption plus renegotiation enables the triple-
handshake attack.  And sending an AUTHENTICATE cell before checking
the CERTS cell would lead to sending the wrong SID field in the
AUTHENTICATE cell, which would whittle our defense there even further.