Handle association proxy delete and provide for scalar delete cascade

Handle association proxy delete and provide for scalar delete cascade

Fixed multiple issues regarding de-association of scalar objects with the
association proxy.  ``del`` now works, and additionally a new flag
:paramref:`.AssociationProxy.cascade_scalar_deletes` is added, which when
set to True indicates that setting a scalar attribute to ``None`` or
deleting via ``del`` will also set the source association to ``None``.

Change-Id: I1580d761571d63eb03a7e8df078cef97d265b85c
Fixes: #4308