Add jitter support for constant back off

[Xuanwo]

Add jitter support for constant back off

[nicoburns]

To elaborate on why this is important:

I once had to deal with a 3rd party API that would fail if it received multiple requests at exactly the same time (or too close together). A simple backoff without jitter would continue to fail because the retries would also be sent at the same time! Adding a random jitter to the backoff meant that the retries would happen at different times, allowing the requests to succeed.