Centralize and sanify generation of synthetic group_by criteria

Centralize and sanify generation of synthetic group_by criteria

This should solve multiple deficiencies when a limited prefetch is ordered
by non-local columns (especially pre-multiplied ones). Also intelligently
solves many problems combining distinct with various non-selecting external
order criteria.

Most current behaviors should remain unaffected except for cases of
blatantly incorrect query generation (fingercross)