Fully implement joining of parent and child mapping classes

Defect Tracker #24384: There is experimental code for joining parent and child mapping classes (as well as unioning sibling queries together) in JoinSourceNodes. However it needs to be refined and tested fully. It should probably be reimplemented so that siblings are joined, not unioned together however due to large result sets this can produce, it would be best to wait for fragments (which should have explicit relationships between parent and child) to take this route.