decodebin2: Don't let thread run after unref

decodebin2: Don't let thread run after unref

We have a dedicated one-shot thread to handle cleanup of old groups.

While this is a good idea. It's an even better idea to make sure
that thread is *completed* before the decodebin2 element to which
it is related isn't freed/gone.

* There can only be one cleanup thread happening at any point in time.
  If there is already one, we wait for the previous one to finish.
* When shutting down (NULL=>READY) make sure the thread is finished

https://bugzilla.gnome.org/show_bug.cgi?id=790007