Shutdown now implicitly occurs if there are no references to a task
and its event queue is empty. The DONEOK flag is forced to true if there are no references to a task, its event queue is empty, and it is shutting down. Add isc_task_unsend() and isc_task_unsendrange(). Add isc_task_sendanddetach(). Event tags are now void *. Code cleanups. Various shutdown bug fixes. Make tracing messages prettier.