[9.18] fix: usr: Fix RPZ race condition during a reconfiguration

With RPZ in use, `named` could terminate unexpectedly because of a race condition when a reconfiguration command was received using `rndc`. This has been fixed.

Closes #5146

Backport of MR !10079

Merge branch 'backport-5146-rpz-reconfig-bug-fix-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10145