- backport of r6540

- backport of r6540
- The "use get" behavior of many-to-one relations, i.e. that a
lazy load will fallback to the possibly cached query.get()
value, now works across join conditions where the two compared
types are not exactly the same class, but share the same
"affinity" - i.e. Integer and SmallInteger.  Also allows
combinations of reflected and non-reflected types to work
with 0.5 style type reflection, such as PGText/Text (note 0.6
reflects types as their generic versions).   [ticket:1556]
- types now support an "affinity comparison" operation, i.e.
that an Integer/SmallInteger are "compatible", or
a Text/String, PickleType/Binary, etc.  Part of
[ticket:1556].