Function jittered_retry_after 

fn jittered_retry_after() -> u64

A Retry-After, in seconds, for when no upstream supplied one.

Jittered around RETRY_AFTER_PERIOD so that senders throttled in the same moment don’t all retry in the same later moment, rebuilding the spike that throttled them. The jitter is symmetric, leaving the mean at the base period: spreading retries costs no additional delivery latency on average.