Test attributes for being non-mapped column properties more closely

Test attributes for being non-mapped column properties more closely

Fixed bug in concrete inheritance mapping where user-defined
attributes such as hybrid properties that mirror the names
of mapped attributes from sibling classes would be overwritten by
the mapper as non-accessible at the instance level.  Also
ensured that user-bound descriptors are not implicitly invoked at the class
level during the mapper configuration stage.

Change-Id: I52b84a15c296b14efeaffb72941fc941d1d52c0d
Fixes: #4188