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/from_db_value/tests.py

@@ -0,0 +1,30 @@
+from django.db import connection
+from django.db.models import Max
+from django.test import TestCase
+
+from .models import CashModel, Cash
+
+
+class FromDBValueTest(TestCase):
+    def setUp(self):
+        CashModel.objects.create(cash='12.50')
+
+    def test_simple_load(self):
+        instance = CashModel.objects.get()
+        self.assertIsInstance(instance.cash, Cash)
+
+    def test_values(self):
+        values_list = CashModel.objects.values_list('cash', flat=True)
+        self.assertIsInstance(values_list[0], Cash)
+
+    def test_aggregation(self):
+        maximum = CashModel.objects.aggregate(m=Max('cash'))['m']
+        self.assertIsInstance(maximum, Cash)
+
+    def test_defer(self):
+        instance = CashModel.objects.defer('cash').get()
+        self.assertIsInstance(instance.cash, Cash)
+
+    def test_connection(self):
+        instance = CashModel.objects.get()
+        self.assertEqual(instance.cash.vendor, connection.vendor)