[9.20] chg: usr: Exempt prefetches from the fetches-per-zone and fetches-per-server quotas
Fetches generated automatically as a result of 'prefetch' are now exempt from the 'fetches-per-zone' and 'fetches-per-server' quotas. This should help in maintaining the cache from which query responses can be given.
Closes #4219 (closed)
Backport of MR !9095 (merged)