Is your feature request related to a problem? Please describe.
The current conn::auto::Connection future always internally determines the appropriate protocol version to use. However, this decision should have already been made by ALPN when running over TLS (or via prior knowledge), so the ReadVersion logic ends up adding some overhead and allows bad clients to use an HTTP version they didn't negotiate.

Describe the solution you'd like
I'd like a version of Connection that's essentially just an enum of the h1 and h2 Connection futures and has to be explicitly told which to use, like the http2_only and http1_only methods in 0.14.