Use configured props for mapper.attrs, mapper.all_orm_descriptors

Use configured props for mapper.attrs, mapper.all_orm_descriptors

Fixed bug where the :attr:`.Mapper.attrs`,
:attr:`.Mapper.all_orm_descriptors` and other derived attributes would
fail to refresh when mapper properties or other ORM constructs were
added to the mapper/class after these  accessors were first called.

(also trying different ways to get the changelog to merge cleanly)

Change-Id: Iaecdb4b3d8c3a3b44302a5880476e60a1f4e27d9
Fixes: #3778