[9.20] fix: usr: Fix a serve-stale issue with a delegated zone

When ``stale-answer-client-timeout 0`` option was enabled, it could be ignored
when resolving a zone which is a delegation of an authoritative zone belonging
to the resolver. This has been fixed.

Closes #5275

Backport of MR !10381

Merge branch 'backport-5275-stale-answer-client-timeout-0-and-delegation-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10420