[ofw] use of io work items

[Sean Hefty]

>We need to come to a consensus on what approach to use before adding other
>asynchronous handling to either the existing winverbs driver or a kernel
>winverbs driver.

See http://msdn.microsoft.com/en-us/library/ms810029.aspx

under Driver Threads (this is in reference to IO_WORKITEM usage:

	Drivers should also avoid queuing excessive numbers of work items,
	because tying up the system worker threads can deadlock the system.
	Instead of queuing a separate work item routine for each individual
	operation, the driver should have a single work item routine that
	performs any outstanding work and then exits when there is no more
	immediate work to perform.

Now I'm wondering if winverbs should either allocate its own thread(s) or have a
single pool work items.  (My original patch allocated work items per user.)

- Sean