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/mod.rs

@@ -3,10 +3,12 @@
 cfg_http1! {
     pub(crate) mod h1;
 
-    pub(crate) use self::h1::{Conn, ServerTransaction};
+    pub(crate) use self::h1::Conn;
 
     #[cfg(feature = "client")]
     pub(crate) use self::h1::dispatch;
+    #[cfg(feature = "server")]
+    pub(crate) use self::h1::ServerTransaction;
 }
 
 cfg_http2! {