Strong reference parent object in association proxy

Strong reference parent object in association proxy

Considering the reversal of #597 as well as
84420a1d0fe09d7a45e878e853aa9f5258561f8b as I am unable to reproduce
the original issues from that release.

The long-standing behavior of the association proxy collection maintaining
only a weak reference to the parent object is reverted; the proxy will now
maintain a strong reference to the parent for as long as the proxy
collection itself is also in memory, eliminating the "stale association
proxy" error. This change is being made on an experimental basis to see if
any use cases arise where it causes side effects.

Change-Id: I051334be90a343dd0e8a1f35e072075eb14b14a7
Fixes: #4268