Clean up DSOM backpointers and object-factory clutter

Before the various design notes can be finalized some cleanups have to be done to wrap up DAFFODIL-1444 which is functionally complete.

This ticket is for cleaning up the various factories which insert backpointers, and removal of all the unique-backpointer cruft which in principle isn't used anymore, as well as chasing down those usages and changing them to what they should be, which is using the non-unique backpointers in the Root.refMap.