Add a lock to prevent overlapping of request and release pads, to close a race where multiqueue might try and add a slot with an id that hasn't quite finished being removed yet by another thread. Fix for https://gitlab.freedesktop.org/bilboed/gstreamer/-/issues/5 and https://gitlab.freedesktop.org/bilboed/gstreamer/-/issues/11 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2784>