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

Merge branch '4850-add-an-additional-class-of-names-to-adb' into 'main'

See merge request isc-projects/bind9!9314