Add an http1_only configuration for the server

[seanmonstar]

The default server now supports both HTTP/1 and 2 automatically. Just like there is an option to set http2_only, there should probably also be an http1_only option, if for some reason someone didn't want to allow HTTP2 support.

[ubnt-intrepid]

When the http1_only mode is added, the behavior when the argument of http2_only(enabled) is false becomes undefined (there are two possible modes whether the fallback is required or not).

It seems to be better to separate the configuration for the protocol (using HTTP/1 or HTTP/2) and upgrading (to enable/disable the fallback to h2 when using h1).

[seanmonstar]

I actually think all cases make sense, even though they may conflict with each other. Trying to set http1_only(true) and http2_only(true) would of course be invalid. It could be asserted, or it could simply take the latter call.

Though, separating them could also work.

[ubnt-intrepid]

I've just created an PR for resolving this issue, just adding http1_only to Connection and Builder.

The flag field http2 in the struct Http has replaced with an enum which has three modes (http1_only, http2_only and fallback).
When http2_only(false) or http1_only(false) are called, the inner mode is set to "fallback".