The window and cooldown resets set attempts to 1. The gate then counted
the same call again, so a reset window allowed one call fewer than a
fresh one. With a limit of 1 the session locked out at once. Resets now
start at 0.
The half-open breaker admitted every caller. It now admits one probe,
claimed with a compare-and-swap. A stale probe is replaced after
OpenDuration, so a lost probe cannot wedge the breaker.
