Backport the split server conn modules

[seanmonstar]

In 1.0, we've split hyper::server::conn::Connection into per-version types. To ease upgrading (see #3052), we can backport the addition of the two modules, hyper::server::conn::{http1, http2}. With them in place, we could then add a deprecation to hyper::server::conn::Connection.

This is very similar in scope to #3053

[seanmonstar]

This was done in #3102.