[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.

Merge branch 'ondrej/dont-run-more-events-than-scheduled-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9257