Remember parent NS set, to be able to fallback to it if needed

Remember parent NS set, to be able to fallback to it if needed

When a non-auth NS set is replaced by an auth one, remember the non-auth one if
it contains NS not in the auth set.

Use that NS set later as a fallback if resolving fails.

TODO: general validation of method, pruning the new table, tests, config switch?

Should fix #10594