Remove an ask's original only when its sequence still holds it, and only with a worker (issue 334)

A seat's queue made again numbers from one, so an old dead letter's sequence
can name a live ask; deleting by number alone would drop it silently. An ask
with no worker would wait unseen while counted as delivered.

cmd/mesh-controller/deadletters.go

@@ -144,11 +144,15 @@ func actOnDeadLetter(ctx context.Context, on *busHandles, act string, id uint64,
 	}
 	switch act {
 	case "deliver":
-		_, to, err := link.DeliverAgain(on.js, id)
+		delivered, to, err := link.DeliverAgain(on.js, id)
 		if err != nil {
 			return nil, err
 		}
 		answer["delivered_on"] = to
+		if delivered.Original != "" {
+			// What became of the ask in its seat's queue (novox/hq issue 334): removed, or left, and why.
+			answer["original"] = delivered.Original
+		}
 		answer["done"] = fmt.Sprintf("dead letter %d was delivered again to %s, and nobody else; it is no longer kept",
 			id, consumerWho(d.Stream, d.Consumer))
 	case "drop":