This adds a method to the DB API that lets us query out the next
instance for a sort query, but without a marker UUID. If the marker
is from another cell, we'll need to resume the pagination sequence
based on the sort keys and the values from the marker instance.
This adds the method and a test to be used later.
Change-Id: Ib0e63ec0dd12c2feff562311bd7f69234cf43a1b