Remove isc_task_onshutdown()

The isc_task_onshutdown() was used to post event that should be run when the task is being shutdown. This could happen explicitly in the isc_test_shutdown() call or implicitly when we detach the last reference to the task and there are no more events posted on the task.

This whole task onshutdown mechanism just makes things more complicated, and it's easier to post the "shutdown" events when we are shutting down explicitly and the existing code already always knows when it should shutdown the task that's being used to execute the onshutdown events.

Replace the isc_task_onshutdown() calls with explicit calls to execute the shutdown tasks.