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/models.py

@@ -0,0 +1,32 @@
+import decimal
+
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+
+
+class Cash(decimal.Decimal):
+    currency = 'USD'
+
+    def __str__(self):
+        s = super(Cash, self).__str__(self)
+        return '%s %s' % (s, self.currency)
+
+
+class CashField(models.DecimalField):
+    def __init__(self, **kwargs):
+        kwargs['max_digits'] = 20
+        kwargs['decimal_places'] = 2
+        super(CashField, self).__init__(**kwargs)
+
+    def from_db_value(self, value, connection):
+        cash = Cash(value)
+        cash.vendor = connection.vendor
+        return cash
+
+
+@python_2_unicode_compatible
+class CashModel(models.Model):
+    cash = CashField()
+
+    def __str__(self):
+        return str(self.cash)