[9.18] fix: dev: Don't loop indefinitely when isc_task quantum is 'unlimited'

Don't run more events than already scheduled. If the quantum is set to a high value, the task_run() would execute already scheduled, and all new events that result from running event->ev_action().

Setting quantum to a number of scheduled events will postpone events scheduled after we enter the loop here to the next task_run() invocation.