Unify subquery generation in the ORM

[Anssi Kääriäinen]

There are some different ways for doing subqueries in the ORM currently. Query.split_exclude(), subqueries in deletion, as_nested_sql() and SubqueryConstraint seem to do something similar. Sometimes there are duplicated code paths, sometimes upper layers do something the lower layers will repeat later on.

I think every instance where a subquery constraint is needed should use the SubqueryConstraint class either directly, or through QuerySet.filter() or sql.Query.build_filter(). This will allow doing all the somewhat complicated setup in one place.