Update current timestamp on each driver loop iteration

Ensures a reasonable upper bound for error.

quinn/src/connection.rs

@@ -90,8 +90,8 @@ impl Future for ConnectionDriver {
     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
         let conn = &mut *self.0.lock().unwrap();
 
-        let now = Instant::now();
         loop {
+            let now = Instant::now();
             let mut keep_going = false;
             conn.process_conn_events().unwrap();
             conn.drive_transmit(now);

quinn/src/endpoint.rs

@@ -129,8 +129,8 @@ impl Future for EndpointDriver {
         if endpoint.driver.is_none() {
             endpoint.driver = Some(task::current());
         }
-        let now = Instant::now();
         loop {
+            let now = Instant::now();
             let mut keep_going = false;
             keep_going |= endpoint.drive_recv(now)?;
             endpoint.drive_incoming();