readyAt/readyMatch belong to the exited generation but were only reset by
#launch, which runs after the restart backoff delay. During the "restarting"
window the sticky-marker predicate from the prior commit therefore reported a
dead service as ready, letting start and for:"ready" waits race it. Clear both
markers when #settle enters "restarting"; #launch re-sets them once the new
child is up. Adds a regression test that observes the backoff window.
Fixes#6303
roboomp
4a000330c6