- _get_equivalents() converted into a lazy-initializing property; Query was calling it

- _get_equivalents() converted into a lazy-initializing property; Query was calling it
for polymorphic loads which is really expensive
- surrogate_mapper adapts the given order_by, so that order_by can be against the mapped
table and is usable for sub-mappers as well.  Query properly calls select_mapper.order_by.