Fixed #18757, #14462, #21565 -- Reworked database-python type conversions

Complete rework of translating data values from database

Deprecation of SubfieldBase, removal of resolve_columns and
convert_values in favour of a more general converter based approach and
public API Field.from_db_value(). Now works seamlessly with aggregation,
.values() and raw queries.

Thanks to akaariai in particular for extensive advice and inspiration,
also to shaib, manfre and timograham for their reviews.

docs/internals/deprecation.txt

@@ -57,6 +57,8 @@ about each item can often be found in the release notes of two versions prior.
 * The ``is_admin_site`` argument to
   ``django.contrib.auth.views.password_reset()`` will be removed.
 
+* ``django.db.models.field.subclassing.SubfieldBase`` will be removed.
+
 .. _deprecation-removed-in-1.9:
 
 1.9