feat(server): server::conn::http1 and server::conn::http2 modules (#3011)

This creates submodules of `hyper::server::conn` for HTTP/1 and HTTP/2.
Each module contains a `Builder` and `Connection`, with options and
methods specific to their version.

Closes #2851 

Co-authored-by: Michael J Ward <[email protected]>

Author: seanmonstar