feat(http1): Add higher-level HTTP upgrade support to Client and Server (#1563)

- Adds `Body::on_upgrade()` that returns an `OnUpgrade` future.
- Adds `hyper::upgrade` module containing types for dealing with
  upgrades.
- Adds `server::conn::Connection::with_upgrades()` method to enable
  these upgrades when using lower-level API (because of a missing
  `Send` bound on the transport generic).
- Client connections are automatically enabled.
- Optimizes request parsing, to make up for extra work to look for
  upgrade requests.
  - Returns a smaller `DecodedLength` type instead of the fatter
    `Decoder`, which should also allow a couple fewer branches.
  - Removes the `Decode::Ignore` wrapper enum, and instead ignoring
    1xx responses is handled directly in the response parsing code.

Ref #1563 

Closes #1395

src/proto/h2/server.rs

@@ -7,6 +7,7 @@ use ::body::Payload;
 use ::common::Exec;
 use ::headers;
 use ::service::Service;
+use ::proto::Dispatched;
 use super::{PipeToSendStream, SendBuf};
 
 use ::{Body, Response};
@@ -82,7 +83,7 @@ where
     S::Future: Send + 'static,
     B: Payload,
 {
-    type Item = ();
+    type Item = Dispatched;
     type Error = ::Error;
 
     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
@@ -95,12 +96,13 @@ where
                     })
                 },
                 State::Serving(ref mut srv) => {
-                    return srv.poll_server(&mut self.service, &self.exec);
+                    try_ready!(srv.poll_server(&mut self.service, &self.exec));
+                    return Ok(Async::Ready(Dispatched::Shutdown));
                 }
                 State::Closed => {
                     // graceful_shutdown was called before handshaking finished,
                     // nothing to do here...
-                    return Ok(Async::Ready(()));
+                    return Ok(Async::Ready(Dispatched::Shutdown));
                 }
             };
             self.state = next;