- The "foreign_keys" argument of relation() will now propagate

- The "foreign_keys" argument of relation() will now propagate
automatically to the backref in the same way that
primaryjoin and secondaryjoin do.   For the extremely
rare use case where the backref of a relation() has
intentionally different "foreign_keys" configured, both sides
now need to be configured explicity (if they do in fact require
this setting, see the next note...).

- ...the only known (and really, really rare) use case where a
different foreign_keys setting was used on the forwards/backwards
side, a composite foreign key that partially points to its own
columns, has been enhanced such that the fk->itself aspect of the
relation won't be used to determine relation direction.