[9.20] fix: usr: Separate DNSSEC validation from the long-running tasks
As part of the KeyTrap [CVE-2023-50387] mitigation, the DNSSEC CPU-intensive operations were offloaded to a separate threadpool that we use to run other tasks that could affect the networking latency.
If that threadpool is running some long-running tasks like RPZ, catalog zone processing, or zone file operations, it would delay DNSSEC validations to a point where the resolving signed DNS records would fail.
Split the CPU-intensive and long-running tasks into separate threadpools in a way that the long-running tasks don't block the CPU-intensive operations.
Closes #4898 (closed)
Backport of MR !9473 (merged)