Webhook retry deadline

A delivery queue retries a webhook until the receiver accepts it. attempt_results[i] is "ok" or "fail" for the i-th attempt, and attempt_seconds[i] is how long that attempt runs before its verdict arrives. The first attempt starts at second 0. After a failure the queue waits before trying again: first_backoff seconds after the first failure, then twice the previous wait each time, never longer than max_backoff. The queue abandons the delivery when an attempt would start later than give_up_after seconds. An attempt already running is never cut short, so a success can land after that deadline. Return the second at which the accepting attempt returns, or -1 when the queue abandons the delivery or runs out of listed attempts. The two lists have the same length, every duration is at least 0, and first_backoff is at most max_backoff.