Type Alias DeferredState

What a caller should do with a deferred request right now (the first request never has the answer; the queue drains on an alarm).

• miss nothing queued; queue it and come back
• pending queued, not yet drained; come back
• ready a fresh result is available; consume it
• expired a result existed but is too old; re-queue rather than serve it
• failed the attempt budget is spent; a definite no (stops a form retrying forever)