* Removed all hardcoded logic for _{fieldname}_cache.
* Added an internal API for interacting with the field values cache.

Thanks carljm and MarkusH for support.

Refs #28459 -- Made Oracle get_db_converters() return converter for empty strings only when it's needed.

Follow up to 8f6a1a15.

It's unclear if the original change was needed, but it seems unneeded now.
Reverted 6fc10f50.

Thanks Tim Graham for the review.

Tested that the extra select that appears in the select clause only because of
distinct() and order_by() doesn't appear in the result values.

Thanks Tim Graham for the review.

Developed by Timothy Allen and Chad Whitman of The Wharton School with
shepherding from Aymeric Augustin and Collin Anderson.

Obsolete since fae56427.

Unused since f59fd15c (Combinable) and
since its introduction in 2d877da8
(SearchVectorCombinable/SearchQueryCombinable/SearchQuery).

Tests were omitted in the original commit: a56a2262.

The test passes before and after the removal. unicode_hint will never
be SafeText, so normal autoescaping is sufficient.

Unused since 9eb2afdd.

Also fixed crash of .filter(field__transform=None).