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)

tests/model_inheritance/tests.py

@@ -294,7 +294,7 @@ class ModelInheritanceTests(TestCase):
             rating=4,
             chef=c
         )
-        with self.assertNumQueries(6):
+        with self.assertNumQueries(3):
             ir.save()
 
     def test_update_parent_filtering(self):