feat(http2): Make HTTP/2 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/2 methods and support behind an
  `http2` cargo feature, which will not be enabled by default. To use
  HTTP/2, add `features = ["http2"]` to the hyper dependency in your
  `Cargo.toml`.

src/client/connect/mod.rs

@@ -184,6 +184,7 @@ impl Connected {
 
     // Don't public expose that `Connected` is `Clone`, unsure if we want to
     // keep that contract...
+    #[cfg(feature = "http2")]
     pub(super) fn clone(&self) -> Connected {
         Connected {
             alpn: self.alpn.clone(),