async-client: Allow sending requests via a &Client

This allows concurrent usage of the client, e.g. by using an `Arc<Client>`.

Signed-off-by: Chen Yu <[email protected]>

src/asynchronous/client.rs

@@ -147,7 +147,7 @@ impl Client {
         Client { req_tx }
     }
 
-    pub async fn request(&mut self, req: Request) -> Result<Response> {
+    pub async fn request(&self, req: Request) -> Result<Response> {
         let mut buf = Vec::with_capacity(req.compute_size() as usize);
         {
             let mut s = CodedOutputStream::vec(&mut buf);