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

Merge branch '4219-exempt-good-queries-from-fetch-limits' into 'main'

Closes #4219

See merge request isc-projects/bind9!9095