This function was supposed to implement a half-duplex mode for our
TLS connections. However, nothing in Tor actually uses it (besides
some unit tests), and the implementation looks really questionable
to me. It's probably best to remove it. We can add a tested one
later if we need one in the future.
Also, add a stubbed-out nss version of the modules. The tests won't
pass with NSS yet since the NSS modules don't do anything.
This is a good patch to read with --color-moved.
