feat(client): Make client an optional feature

cc #2223

BREAKING CHANGE: The HTTP client of hyper is now an optional feature. To
  enable the client, add `features = ["client"]` to the dependency in
  your `Cargo.toml`.

src/proto/h1/mod.rs

@@ -19,7 +19,10 @@ mod io;
 mod role;
 
 pub(crate) type ServerTransaction = role::Server;
-pub(crate) type ClientTransaction = role::Client;
+
+cfg_client! {
+    pub(crate) type ClientTransaction = role::Client;
+}
 
 pub(crate) trait Http1Transaction {
     type Incoming;