Remove the buffer queue.

A client calling attach(A);commit();attach(B);commit() should result
in its back buffer be set to B, and A be discarded.
If the client wants to have a queue it should keep it client side or
a protocol extension tailored for that purpose should be developed.

Change-Id: Ia0048f311504d85821df9f5b9225887801efec71
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>