iou: introduce iou_async() for threading

This is a preliminary implementaion of an iou-abstracted thread
pool.

Two callbacks are provided to iou_async():

async_cb:
 Called by a worker thread and must be carefully written for
 concurrent execution.

completion_cb:
 Called by iou_run() after a thread has finished with async_cb().

As completion_cb() is called in the iou_run() context, assuming
the caller is always entering iou_run() from the main thread,
it may be viewed as a continuation point for the async work
reentering the serial execution flow.

At this time every iou_new() will create two worker threads.  In
the future it may just create a thread per core or something like
that.

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