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/field_subclassing/models.py

@@ -2,13 +2,24 @@
 Tests for field subclassing.
 """
 
+import warnings
+
 from django.db import models
 from django.utils.encoding import force_text
+from django.utils.deprecation import RemovedInDjango20Warning
 
 from .fields import Small, SmallField, SmallerField, JSONField
 from django.utils.encoding import python_2_unicode_compatible
 
 
+# Catch warning about subfieldbase  -- remove in Django 2.0
+warnings.filterwarnings(
+    'ignore',
+    'SubfieldBase has been deprecated. Use Field.from_db_value instead.',
+    RemovedInDjango20Warning
+)
+
+
 @python_2_unicode_compatible
 class MyModel(models.Model):
     name = models.CharField(max_length=10)