Fixed #16649 -- Refactored save_base logic

Took UPDATE - if not updated - INSERT, instead of SELECT - if found
UPDATE else INSERT into use for model.save(). This should save a
query in majority of cases.

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)