Caching in inheritance accessor functions

[mldwg]

The accessor functions for inheritance relations are overwritten in PolymorphicModel. This also disables the caching usually provided by django, creating multiple redundant SQL queries in certain situations:

parent = Parent.object.get(id=1)
c1 = parent.child  # retrieves child from db
c2 = parent.child  # retrieves child from db again!

This contradicts the default django behavior and can cause unnecessary overhead.

This PR fixes that issue by using the caching mechanism of the underlying field.
The new unit test test_polymorphic__accessor_caching validates this new feature.