feat(http1): Make HTTP/1 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/1 methods and support behind an
  `http1` cargo feature, which will not be enabled by default. To use
  HTTP/1, add `features = ["http1"]` to the hyper dependency in your
  `Cargo.toml`.

src/client/dispatch.rs

@@ -162,11 +162,13 @@ impl<T, U> Receiver<T, U> {
         }
     }
 
+    #[cfg(feature = "http1")]
     pub(crate) fn close(&mut self) {
         self.taker.cancel();
         self.inner.close();
     }
 
+    #[cfg(feature = "http1")]
     pub(crate) fn try_recv(&mut self) -> Option<(T, Callback<T, U>)> {
         match self.inner.try_recv() {
             Ok(mut env) => env.0.take(),