Add exponential backoff with jitter

[mononym]

This is a more dynamic and smooth backoff strategy than a simple exponential or even exponential piped through the randomizer, both of which will create groupings of calls at certain points in time.

See the link provided in the documentation for the function for more detailed info on the algorithm and effects.

[safwank]

Thanks for the PR @mononym. I'm wondering if it makes more sense to introduce a new function jitter/1 (similar to randomize/2) to which you can pipe a stream of exponential delays, e.g.

def jitter(delays) do
  Stream.map(delays, fn d ->
    :rand.uniform(trunc(d))
  end)
end

That way the randomization function is decoupled from the exponentiation function to preserve their composability.

What do you reckon?

[mononym]

Should work just fine. Made the adjustment.