feat(lib): add support to disable tokio-proto internals

For now, this adds `client::Config::no_proto`, `server::Http::no_proto`,
and `server::Server::no_proto` to skip tokio-proto implementations, and
use an internal dispatch system instead.

`Http::no_proto` is similar to `Http::bind_connection`, but returns a
`Connection` that is a `Future` to drive HTTP with the provided service.
Any errors prior to parsing a request, and after delivering a response
(but before flush the response body) will be returned from this future.

See #1342 for more.

examples/hello.rs

@@ -31,7 +31,8 @@ impl Service for Hello {
 fn main() {
     pretty_env_logger::init().unwrap();
     let addr = "127.0.0.1:3000".parse().unwrap();
-    let server = Http::new().bind(&addr, || Ok(Hello)).unwrap();
+    let mut server = Http::new().bind(&addr, || Ok(Hello)).unwrap();
+    server.no_proto();
     println!("Listening on http://{} with 1 thread.", server.local_addr().unwrap());
     server.run().unwrap();
 }