Handle new query types once core 297 is merged

[bserdar]

Once lightblue-platform/lightblue-core#297 is merged, the back-end code should handle the new query types: NaryValueRelationalExpression and NaryFieldRelationalExpression instead of NaryRelationalExpression. One way to make sure the code remains aware of the changes is to make sure you use QueryIterator when processing queries, instead of manually descending the query tree.