Give each asking of a merge ask its own id, and add the have phase removed (issues 1370, 1369)

The router drops an ask under an id it keeps as a redelivery, so a gate that writes the same group's ask again at unchanged heads was never heard. Ask gains Asking; AskingID gives the first asking the old id and later ones their own. PhaseRemoved lets the node-engine say a module left its node.

go/delivery/gate.go

@@ -119,6 +119,18 @@ func AskID(askKey string) string {
 	return string(readable) + "-" + hex.EncodeToString(sum[:])[:12]
 }
 
+// AskingID is the id of the asking-th writing of the ask at askKey. Asking 0 and 1 give AskID(askKey), so the first
+// asking's id is unchanged; asking n of 2 or more gives the AskID form of askKey + "." + n, so each asking has an id
+// of its own (at most 64 bytes, ending in the 12 hex digits of the hash of that whole string) and the router, which
+// drops an ask under an id it keeps as a redelivery, takes each as new and an answer to an earlier asking is never
+// taken for a later one.
+func AskingID(askKey string, asking int) string {
+	if asking < 2 {
+		return AskID(askKey)
+	}
+	return AskID(askKey + "." + strconv.Itoa(asking))
+}
+
 func checkOwedAct(act string) (kind, context string, err error) {
 	kind, context, hasContext := strings.Cut(act, "=")
 	switch kind {
@@ -274,15 +286,22 @@ type Ask struct {
 	Group string   `json:"group"`
 	Heads []Head   `json:"heads"`
 	Ask   asks.Ask `json:"ask"`
+	// Asking counts the times the gate wrote this group's ask at these heads: 0 or 1 for the first, n for the n-th.
+	// The router drops an ask whose id it has already kept as a redelivery, so each asking has its own id
+	// (AskingID); the first asking's id is unchanged and the field is omitted for it.
+	Asking int `json:"asking,omitempty"`
 }
 
 func (a Ask) check() error {
+	if a.Asking < 0 {
+		return fmt.Errorf("delivery: the ask's asking %d is negative", a.Asking)
+	}
 	key, err := MergeAskKey(a.Group, a.Heads)
 	if err != nil {
 		return fmt.Errorf("delivery: an ask needs its group and at least one pull request's head: %w", err)
 	}
-	if want := AskID(key); a.Ask.ID != want {
-		return fmt.Errorf("delivery: the ask's id %q is not %q, the id its group and heads give", a.Ask.ID, want)
+	if want := AskingID(key, a.Asking); a.Ask.ID != want {
+		return fmt.Errorf("delivery: the ask's id %q is not %q, the id its group, heads and asking %d give", a.Ask.ID, want, a.Asking)
 	}
 	if len(a.Ask.Options) == 0 {
 		return fmt.Errorf("delivery: the ask has no option")