Sorting on referenced properties is broken

[pcoccoli]

See opencybersecurityalliance/kestrel-lang#435

You can reproduce with something like:

   query = Query([
       Table('conns'),
       Order([('dst_ref.value', Order.ASC)]),
   ])
   store.assign_query('sconns', query)
   _ = store.lookup('sconns')

This will fail with firepit.exceptions.InvalidAttr: invalid attribute: sconns.dst_ref.value