HTTP: Improve cookie handling

[Tectu]

The http::request constructor accepting an rvalue of the underlying beast request currently parses cookies. However, this doesn't really make sense. We should only parse cookies when we really need them. This should happen outside of the http::request type. We already have http::cookie_value for this.

Furthermore, the cookie handling between an http::request and an http::response should be the same. The same free standing functions can be used.