Add isc_task_setquantum() and use it for post-init zone loading

Add isc_task_setquantum() and use it for post-init zone loading

Add isc_task_setquantum() function that modifies quantum for the future
isc_task_run() invocations.

NOTE: The current isc_task_run() caches the task->quantum into a local
variable and therefore the current event loop is not affected by any
quantum change.