remove intermediary _is_clone_of entries when cloning

remove intermediary _is_clone_of entries when cloning

Improvements in memory usage by the ORM, removing a significant set of
intermediary expression objects that are typically stored when a copy of an
expression object is created. These clones have been greatly reduced,
reducing the number of total expression objects stored in memory by
ORM mappings by about 30%.

note this change causes the tests to have a bit of a harder time with
GC, which we would assume is because mappings now have a lot more
garbage to clean up after mappers are configured.   it remains
to be seen what the long term effects of this are.

Fixes: #7823
Change-Id: If8729747ffb9bf27e8974f069a994b5a823ee095