[WIP] Split server conn types

[Michael-J-Ward]

Closes #2851

If someone takes a look and provides some guidance, I'll execute the next steps.

TODO:

• create separate http1 type
• create separate http2 type
• configuring / constructing?
• excise old server::conn::Connection

[seanmonstar]

Thanks for getting started with this! You said you think next steps are configuration? The Http type could likely be split into a Builder in each module, only including the relevant version methods. Similar to how it is in client::conn.

[seanmonstar]

Thank you for the big start! I've finished up the split in #3011.