Repair isc_task_purgeevent(), clean isc_task_unsend{,range}()
The isc_task_purgerange() was walking through all events on the task to find a matching task. Instead use the ISC_LINK_LINKED to find whether the event is active.
Cleanup the related isc_task_unsend() and isc_task_unsendrange() functions that were not used anywhere.
Closes: #3252 (closed)
Edited by Ondřej Surý