WeakCompositeKey was coded incorrectly and was not weakly referencing anything. ...

WeakCompositeKey was coded incorrectly and was not weakly referencing anything.  However when repaired, the usage within RelationLoader._create_joins() still creates cycles between key elements and the value placed in the dict.  In the interests of risk reduction, WCK is now removed and the two caches it was used for are now non-cached.  Speed comparisons with one join/eager-heavy web application show no noticeable effect in response time.