Run the RPZ update as offloaded work

Previously, the RPZ updates ran quantized on the main nm_worker loops. As the quantum was set to 1024, this might lead to service interruptions when large RPZ update was processed.

Change the RPZ update process to run as the offloaded work. The update and cleanup loops were refactored to do as little locking of the maintenance lock as possible for the shortest periods of time and the db iterator is being paused for every iteration, so we don't hold the rbtdb tree lock for prolonged periods of time.

Closes #3190 (closed)