Committing Avination's memleak fix-a-thon, installment #3

When linking, detach the no longer used SOG's from backup so they can be
collected. Since their Children collection is never emptied, they prevent
their former SOPs from being collected as well.

OpenSim/Region/Framework/Scenes/SceneGraph.cs

@@ -1638,6 +1638,8 @@ namespace OpenSim.Region.Framework.Scenes
                     {
                         parentGroup.LinkToGroup(child);
 
+                        child.DetachFromBackup();
+
                         // this is here so physics gets updated!
                         // Don't remove!  Bad juju!  Stay away! or fix physics!
                         child.AbsolutePosition = child.AbsolutePosition;