Run event / notification handlers in separate async tasks

Prevents deadlocks when new handlers are registered from within an
existing handler.

matrix_sdk/src/client.rs

@@ -2123,7 +2123,7 @@ impl Client {
                 };
 
                 for notification in room_notifications {
-                    (handler)(notification.clone(), room.clone(), self.clone()).await;
+                    tokio::spawn((handler)(notification.clone(), room.clone(), self.clone()));
                 }
             }
         }

matrix_sdk/src/event_handler.rs

@@ -300,7 +300,7 @@ impl Client {
                         room: room.clone(),
                         raw: event.json(),
                     };
-                    (handler)(data).await;
+                    tokio::spawn((handler)(data));
                 }
             }
         }