Break association proxy into a descriptor + per-class accessor

Break association proxy into a descriptor + per-class accessor

Reworked :class:`.AssociationProxy` to store state that's specific to a
parent class in a separate object, so that a single
:class:`.AssocationProxy` can serve for multiple parent classes, as is
intrinsic to inheritance, without any ambiguity in the state returned by it.
A new method :meth:`.AssociationProxy.for_class` is added to allow
inspection of class-specific state.

Change-Id: I634f88aae6306ac5c5237a0e1acbe07d0481d6b6
Fixes: #3423