[9.20] fix: usr: preserve cache when reload fails and reload the server again

Fixes an issue where failing to reconfigure/reload the server would prevent to preserved the views caches on the subsequent server reconfiguration/reload.

Closes #5523

Backport of MR !10984

Merge branch 'backport-colin/fix-cache-revert-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10988