[Bug] Ordering by children can be nondeterministic

[lutter]

Bug report

With a schema like

type Parent @entity {
  id: Bytes!
 child: Child!
}

type Child @entity {
  id: Bytes!
}

the result for a query like

query { parents(orderBy: child__id) } { id } }

is nondeterministic if multiple parents point to the same child. To make this entirely clear, assume there is only one instance of Child. The SQL query that gets generated is roughly

select * 
  from parent p 
    left join child c on (c.id = p.child) 
where ...
order by c.id

But when multiple rows in p have the same p.child, that ordering is ill-defined. We'd need to change that to order by c.id, p.id to make that deterministic.