Use non-blocking sleep function

std::thread::sleep blocks the entire thread and thus also forces all other current requests to be halted.

src/results/aggregator.rs

@@ -74,7 +74,7 @@ pub async fn aggregate(
     if random_delay || !debug {
         let mut rng = rand::thread_rng();
         let delay_secs = rng.gen_range(1..10);
-        std::thread::sleep(Duration::from_secs(delay_secs));
+        tokio::time::sleep(Duration::from_secs(delay_secs)).await;
     }
 
     // fetch results from upstream search engines simultaneously/concurrently.