Run RPZ and catalog zones tasks in exclusive mode

Run RPZ and catalog zones tasks in exclusive mode

All the heavy RPZ and CATZ work is already running with offloaded
threads, and running the remaining small functions in exclusive mode
offers more synchronization guaranties.

Move the update notify registration code from the offloaded
dns__catz_update_cb() function into dns__catz_done_cb().

After this change, it should be safe to remove the lock/unlock code
from the dns_catz_dbupdate_register() and dns_catz_dbupdate_unregister()
functions, as they were causing a benign TSAN lock-order-inversion
report.