Changed verified event names - cmpute.instance.verified.old to compute.instance.verified and image.exists.verified.old to image.exist.verified
Fixed race condition which was causing multiple verified messages to be sent by
starting the async verify process per owner instead of per image.
Made changes so that we verify each group of exists messages with
the same owner and raw_id separately. This ensures that if there are
multiple exists notifications for each owner pending verification,
we send an individual verified notification for each of them.