Remove task privileged mode

Previously, the task privileged mode has been used only when the named was starting up and loading the zones from the disk as the "first" thing to do. The privileged task was setup with quantum == 2, which made the taskmgr/netmgr spin around the privileged queue processing two events at the time.

The same effect can be achieved by setting the quantum to UINT_MAX (e.g. practically unlimited) for the loadzone task, hence the privileged task mode was removed in favor of just processing all the events on the loadzone task in a single task_run().

Closes: #3253 (closed)