[9.20] fix: usr: Fix intermittent named crashes during asynchronous zone operations

Asynchronous zone loading and dumping operations occasionally dispatched tasks
to the wrong internal event loop. This threading violation triggered internal
safety assertions that abruptly terminated named. Strict loop affinity is now
enforced for these tasks, ensuring they execute on their designated threads
and preventing the crashes.

Closes #4882

Backport of MR !11655

Merge branch 'backport-4882-run-rndc-zone-commands-on-correct-loop-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11684