Yield to the executor in the the Endpoint

As long as there is data to send or receive the `Endpoint` `Future`
will currently continue to execute and thereby retain the eventloop.

In order to allow other code to execute in between we yield back
to the executor after each iteration. Instead of doing mulitiple
iterations in one `EndpointDriver::poll` call we can just increase the
maximum amount of packets to send or receive in a single iteration
and pick a number which optimizes performance.
Therefore the loop gets completely removed.

Author: Matthias247

quinn/src/endpoint.rs

@@ -200,28 +200,33 @@ where
 
     #[allow(unused_mut)] // MSRV
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-        let endpoint = &mut *self.0.lock().unwrap();
+        let mut endpoint = self.0.lock().unwrap();
         if endpoint.driver.is_none() {
             endpoint.driver = Some(cx.waker().clone());
         }
-        loop {
-            let now = Instant::now();
-            let mut keep_going = false;
-            keep_going |= endpoint.drive_recv(cx, now)?;
-            endpoint.handle_events(cx);
-            keep_going |= endpoint.drive_send(cx)?;
-            if !keep_going {
-                break;
-            }
-        }
+
+        let now = Instant::now();
+        let mut keep_going = false;
+        keep_going |= endpoint.drive_recv(cx, now)?;
+        endpoint.handle_events(cx);
+        keep_going |= endpoint.drive_send(cx)?;
+
         if !endpoint.incoming.is_empty() {
             if let Some(task) = endpoint.incoming_reader.take() {
                 task.wake();
             }
         }
+
         if endpoint.ref_count == 0 && endpoint.connections.is_empty() {
             Poll::Ready(Ok(()))
         } else {
+            drop(endpoint);
+            // If there is more work to do schedule the endpoint task again.
+            // `wake_by_ref()` is called outside the lock to minimize
+            // lock contention on a multithreaded runtime.
+            if keep_going {
+                cx.waker().wake_by_ref();
+            }
             Poll::Pending
         }
     }