Add http1_writev config for Server

[seanmonstar]

There is an http1_writev config option in the client to disable writev usage internally. While the default is to use auto-detection of writev, the detection must happen on every connection, and for shorter lived connections, it'd be better to configure it off from the start.

This issue is just to add a same kind of config for the server side, by adding hyper::server::conn::Http::http1_writev(enabled: bool). This can trivially be backported for 0.11.