fix: usr: Avoid unnecessary locking in the zone/cache database

Prevent lock contention among many worker threads referring to the same database node at the same time.  This would improve zone and cache database performance for the heavily contended database nodes.

Closes #5130

Merge branch '5130-reduce-lock-contention-in-decrement-reference' into 'main'

See merge request isc-projects/bind9!9963