This patch introduces more retry backoff function the delivery
retry policy into Zaqar. It will work when Zaqar failed to send
the notification to the subscriber. Users can define the retry
backoff function in the options of subscription or metadata
of queue.
Change-Id: I0bffd9249f8a0d466ecea1ac36d8adc37b742238
Implement: blueprint support-more-backoff-functions