Subqueries don't use custom queryset

[Ivan Illarionov]

All classes in django/db/models/sql/subqueries.py subclass directly from Query class even if database backend has uses_custom_queryset feature. This makes as_sql() methods of these classes handle limits incorrectly with backends (eg oracle) that use different constructs.

Classes in django/db/models/sql/subqueries.py should sublass from backend-specific Query class the same way QuerySet class does.

[Ivan Illarionov]

The fix extracted from external backend implementation

[Ivan Illarionov]

Proposal: since all features used by custom QuerySet class were moved to Query class in qsrf branch it makes sense to rename uses_custom_queryset feature to uses_custom_query and make only the Query class customizable. QuerySet class could check for this feature and create its query attribute based on it.

[Malcolm Tredinnick]

(In [7426]) queryset-refactor: Querysets no longer need to be customised per-backend.
Instead, it's the Query class that is backend-dependent. So make that explicit
in the code (code should refer to sql.Query and they will always get the right
backend-specific class).

This also fixes the main part of #6956, in that the various subclasses of Query
automatically use any custom Query class.

Fixed #6956.

[Malcolm Tredinnick]

(In [7427]) queryset-refactor: The Oracle changes necessary for [7426]. I can't test these
at the moment, but they should be close to correct.

Refs #6956.