fix: usr: Fix assertion failure when dumping recursing clients

Previously, if a new counter was added to the hashtable
while dumping recursing clients via the `rndc recursing`
command, and `fetches-per-zone` was enabled, an assertion
failure could occur. This has been fixed.

Closes #5200

Merge branch '5200-destroy-iterator-inside-the-rwlock' into 'main'

See merge request isc-projects/bind9!10164