Fixed #16649 -- Refactored save_base logic

Model.save() will use UPDATE - if not updated - INSERT instead of
SELECT - if found UPDATE else INSERT. This should save a query when
updating, but will cost a little when inserting model with PK set.

Also fixed #17341 -- made sure .save() commits transactions only after
the whole model has been saved. This wasn't the case in model
inheritance situations.

The save_base implementation was refactored into multiple methods.
A typical chain for inherited save is:
save_base()
    _save_parents(self)
        for each parent:
            _save_parents(parent)
            _save_table(parent)
    _save_table(self)

docs/releases/1.6.txt

@@ -150,6 +150,10 @@ Minor features
 * Generic :class:`~django.contrib.gis.db.models.GeometryField` is now editable
   with the OpenLayers widget in the admin.
 
+* The :meth:`Model.save() <django.db.models.Model.save()>` will do
+  ``UPDATE`` - if not updated - ``INSERT`` instead of ``SELECT`` - if not
+  found ``INSERT`` else ``UPDATE`` in case the model's primary key is set.
+
 Backwards incompatible changes in 1.6
 =====================================