Replace Namespace#self_and_descendants with linear version

We are now storing the Namespace path from root_ancestor to self on each Namespace. We will utilize this information to replace each recursive method in !52999 (diffs)

This issue is to replace the Namespace#self_and_descendants with a linear version.

Note that this work has previously been through a significant review process. Unfortunately the MR grew so large that it is now being split in to multiple MRs for the purpose of deployment.

This work will sit behind a use_traversal_ids feature flag.