[Openib-windows] CQ event notification in user-mode

[Sean Hefty]

>Allowing the user to specify a key per CQ implies that each CQ has an
>underlying
>file object, which may impact scalability if a user has many CQs.  Having a
>single key for all completions on the other hand allows for a single CQ
>completion file object (likely managed in the CA object), and the I/O request
>would have to be used to provide per-CQ context using the OVERLAPPED structure
>returned by GetQueuedCompletionStatus (provided when issuing the request).

I'm not sure how Windows implements sockets, but I believe on at least Linux
each socket is associated with a single file object.  If Windows is similar,
then the scalability of having a key per CQ doesn't seem any worse than what an
app can expect from using TCP.  If an app wants better scalability, they can
always use the same CQ with multiple QPs.

- Sean