Extend the queue manager to support notification

[diptorupd]

Currently, all clients of DPPL need to ensure they call DPPLGetCurrentQueue every time they need to use a DPPL queue. A better design will be to allow clients to attach a notification routine that they can use to know when the current queue changes. Needs more thought, but opening the ticket to make sure I do not forget it.

[diptorupd]

I am closing this ticket as in future dpctl will stop providing a global queue manager and this design is never going to be implemented.