[th/gobject-notify-queue] rework locking in `GObject` around `GObjectNotifyQueue`

Access to GData already takes a mutex.

Use g_datalist_id_update_atomic(), which takes a callback and allows to perform more complex operations while holding that lock.

That allows to drop object_bit_lock (object, OPTIONAL_BIT_LOCK_NOTIFY) for locking around GObjectNotifyQueue.

Also, don't use a GSList but reallocate/grow the GObjectNotifyQueue buffer.

---

This is the first part. The follow up is !3864 which reworks more uses of object_bit_lock().