Use plone.batching

Now that Plone 4.3 is out, which ships with plone.batching by default, it's safe to use it in plone.app.querystring. 

(This changeset was previously suggested in https://github.com/plone/plone.app.querystring/pull/3 and https://github.com/plone/plone.app.querystring/pull/5)