[9.20] fix: usr: Fix a bug in the static-stub implementation

Static-stub addresses and addresses from other sources were being
mixed together, resulting in static-stub queries going to addresses
not specified in the configuration, or alternatively, static-stub
addresses being used instead of the correct server addresses.

Closes #4850

Backport of MR !9314

Merge branch 'backport-4850-add-an-additional-class-of-names-to-adb-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9571