feat: drain pending notifications from the client rx queue

Closes #1133

autopush_rs/src/client.rs

@@ -712,7 +712,7 @@ where
 
     pub fn shutdown(&mut self) {
         // If we made it past hello, do more cleanup
-        let webpush = match self.webpush.take() {
+        let mut webpush = match self.webpush.take() {
             Some(webpush) => webpush,
             None => return
         };
@@ -730,6 +730,19 @@ where
             .send()
             .ok();
 
+        // If there's any notifications in the queue, move them to our unacked direct notifs
+        webpush.rx.close();
+        let remaining = webpush.rx.wait();
+        for msg in remaining {
+            match msg {
+                Ok(ServerNotification::CheckStorage) => continue,
+                Ok(ServerNotification::Notification(notif)) => {
+                    webpush.unacked_direct_notifs.push(notif.clone());
+                }
+                Err(_) => continue,
+            }
+        }
+
         // If there's direct unack'd messages, they need to be saved out without blocking
         // here
         self.srv.disconnet_client(&webpush.uaid);