When the camera is being stop()ped, active requests will complete. These
may trigger an application to re-queue those requests to the camera but
that is not permitted, and is an error in the application.

Extend the camera state to include a stopping state which is entered as
soon as a call to stop() is made. At this point, any request queued will
be rejected with a warning, while any pending requests are either
successfully completed or cancelled.

When the pipeline handler has finished stopping, the camera state will
transition to the CameraConfigured state where it can begin to accept
requests again, and be restarted.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>