Fix silverstripe/cms#776 - Change how batch_permission_check works

Will now check stage one after the other, rather than both at once.

Conflicts:
	core/model/SiteTree.php
Sorry, in the standard 2.4 branch it results in buggy behaviour if you leave it on.
I'll fix it since the issue doesn't seem to exist in post-2.4 (though I have no idea when/how it was fixed).