WFS 2.0 cascaded stored query

Support for cascaded stored query backed layers from wfs-ng datastores.
There's only a few places where this signature needs to change, so I wouldn't be too worried about the scope of this change getting out of hand. But I do agree that the constructor is getting a bit out of hand.

Let's see what Andrea thinks. Maybe he can figure out a good pattern to avoid adding more and more things to this constructor.