Jitter algorithm not random once at maxium delay

[fishcakez]

Once the maximum delay is reached using the jitter type the delay is always the maximum. This leads to convergence if multiple processes using backoff delay communicate with a bottleneck. Once one processes gets queued behind another at the bottleneck they remain in sync for the next (maximum) backoff delay (and all future maximum delays). Because the grouping remains for future delays the effect snowballs and more processes join the batch leading to a thundering herd.

Obviously the same effect could occur with the normal type but the jitter algorithm should produce random delays to avoid this.

[fishcakez]

/cc @jpuigm

[jpuigm]

@fishcakez Nice catch, thanks for contributing!