- Two fixes to help prevent out-of-band columns from

- Two fixes to help prevent out-of-band columns from
being rendered in polymorphic_union inheritance
scenarios (which then causes extra tables to be
rendered in the FROM clause causing cartesian
products):
- improvements to "column adaption" for
  a->b->c inheritance situations to better
  locate columns that are related to one
  another via multiple levels of indirection,
  rather than rendering the non-adapted
  column.
- the "polymorphic discriminator" column is
  only rendered for the actual mapper being
  queried against. The column won't be
  "pulled in" from a subclass or superclass
  mapper since it's not needed.