decodebin: aggregate buffering messages

decodebin: aggregate buffering messages

Aggregate buffering messages to only post the lower value
to avoid setting pipeline to playing while any multiqueue
is still buffering.

There are 3 scenarios where the entries should be removed from
the list:

1) When decodebin is set to READY
2) When an element posts a 100% buffering (already implemented)
3) When a multiqueue is removed from decodebin.

For item 3 we don't need to handle it because this should only
happen when either 1 is hapenning or when it is playing a
chained file, for which number 2 should have happened for the
previous stream to finish

https://bugzilla.gnome.org/show_bug.cgi?id=726423