[9.20] fix: usr: Fix a crash when reconfiguring while an NTA is being rechecked

When named was reconfigured or shut down while a negative trust anchor
was being rechecked against authoritative servers, the in-flight recheck
could outlive the view that owned it and cause `named` to crash.  This
has been fixed.

Closes #5938

Backport of MR !11948

Merge branch 'backport-5938-ref-ntatable-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11966