feat(server): change default dispatcher

- Deprecates the `no_proto` configuration on `Server`. It is always
  enabled.
- Deprecates all pieces related to tokio-proto.
- Makes the tokio-proto crate optional, and the `server-proto` feature
  can be used to completely remove the dependency. It is enabled by
  default.

src/client/mod.rs

@@ -198,7 +198,7 @@ where C: Connect,
                     let pooled = pool.pooled(pool_key, tx);
                     let conn = proto::Conn::<_, _, proto::ClientTransaction, _>::new(io, pooled.clone());
                     let dispatch = proto::dispatch::Dispatcher::new(proto::dispatch::Client::new(rx), conn);
-                    handle.spawn(dispatch.map_err(|err| error!("no_proto error: {}", err)));
+                    handle.spawn(dispatch.map_err(|err| error!("client connection error: {}", err)));
                     pooled
                 })
         };