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.

tests/aggregation_regress/tests.py

@@ -894,18 +894,6 @@ class AggregationTests(TestCase):
             lambda b: b.name
         )
 
-    def test_type_conversion(self):
-        # The database backend convert_values function should not try to covert
-        # CharFields to float. Refs #13844.
-        from django.db.models import CharField
-        from django.db import connection
-        testData = 'not_a_float_value'
-        testField = CharField()
-        self.assertEqual(
-            connection.ops.convert_values(testData, testField),
-            testData
-        )
-
     def test_annotate_joins(self):
         """
         Test that the base table's join isn't promoted to LOUTER. This could