fix: usr: Prevent excessive priming queries to the root servers

BIND was sending a priming query to the root servers on nearly every
recursive lookup instead of only when the cached root information
expired.  Priming now rearms only after the TTL of the fetched records
elapses, and the refreshed root NS set is used for query routing until
the next cycle.

Merge branch 'ondrej/fix-delegdb-priming' into 'main'

See merge request isc-projects/bind9!11847