fix: usr: [9.20] Wait for memory reclamation to finish in named-checkconf

Previously, when named-checkzone loaded the zone to the QP database, the delayed
memory reclamation could cause an assertion check on exit. This has
been fixed.

Closes #5220

Merge branch '5220-add-rcu-barrier-to-named-checkconf-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10307