feat(server): Make the server code an optional feature (#2334)

cc #2223 

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

src/proto/h1/io.rs

@@ -66,6 +66,7 @@ where
         }
     }
 
+    #[cfg(feature = "server")]
     pub fn set_flush_pipeline(&mut self, enabled: bool) {
         debug_assert!(!self.write_buf.has_remaining());
         self.flush_pipeline = enabled;