disable HTTP keep-alive

NX seems to freeze after some time when talking to a cache with HTTP
keep-alive enabled

src/main.rs

@@ -28,7 +28,9 @@ async fn main() -> std::io::Result<()> {
                 .service(handlers::get_cache_item)
                 .service(handlers::put_cache_item)
         }
-    });
+    })
+    .keep_alive(None); // disable HTTP keep-alive because it seems to break NX (at least in version 20.8)
+
     for bind_address in config.bind_addresses.unwrap_or(vec!["::0".to_string()]) {
         server = server
             .bind((bind_address.clone(), 8080))