Search through mapper superclass hierarchy for owner

Search through mapper superclass hierarchy for owner

Fixed regression caused by fix for issue :ticket:`4116` affecting versions
1.2.2 as well as 1.1.15, which had the effect of mis-calculation of the
"owning class" of an :class:`.AssociationProxy` as the ``NoneType`` class
in some declarative mixin/inheritance situations as well as if the
association proxy were accessed off of an un-mapped class.  The "figure out
the owner" logic has been replaced by an in-depth routine that searches
through the complete mapper hierarchy assigned to the class or subclass to
determine the correct (we hope) match; will not assign the owner if no
match is found.  An exception is now raised if the proxy is used
against an un-mapped instance.

Change-Id: I611b590df2babe077ce6c19bea89e84251d1a7f4
Fixes: #4185