- Primary key values can now be changed on a joined-table inheritance

- Primary key values can now be changed on a joined-table inheritance
object, and ON UPDATE CASCADE will be taken into account when
the flush happens.  Set the new "passive_updates" flag to False
on mapper() when using SQLite or MySQL/MyISAM. [ticket:1362]

- flush() now detects when a primary key column was updated by
an ON UPDATE CASCADE operation from another primary key, and
can then locate the row for a subsequent UPDATE on the new PK
value.  This occurs when a relation() is there to establish
the relationship as well as passive_updates=True.  [ticket:1671]