Changes I made last week to the task code simplified the task shutdown

and termination model. Here's how things now work:

When a task is shutdown:

Any "on shutdown" events for the task are posted

The "shutting down" attribute of the task is set

Any attempts to add shutdown events with isc_task_onshutdown()

will fail, since the task is already shutting down

Task shutdown can be initiated explicity, via a call to isc_task_shutdown(),

or implicitly, when the following conditions occur:

The "shutting down" attribute of the task is not set

The task has no references

The task has an empty event queue

Task termination occurs when:

The "shutting down" attribute of the task is set

The task has no references

The task has an empty event queue