Change locking around group deactivation to avoid deadlocks
when shutting down exactly as a buffering message arrives.
The PLAYBIN3_LOCK now protects the active field of the
source group. Everything else is still protected by the
source-group-lock.
Also properly protect group switching operations with
the PLAYBIN3_LOCK everywhere.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1049>