A shutdown event specification is no longer part of the
isc_task_create() API.
Multiple shutdown events can be requested with isc_task_onshutdown().
Shutdown events are posted LIFO.
The ability to send events can be enabled and disabled with
isc_task_allowsend().
Event actions return void; the only way to shutdown a task is to
call isc_task_shutdown().
There are no implicit event discards anymore. (There is no need for
them, since event actions cannot request immediate shutdown anymore.)
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