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/delivery_test.go

@@ -292,3 +292,19 @@ func TestSame(t *testing.T) {
 		t.Error("different pulses same")
 	}
 }
+
+func TestHaveRemoved(t *testing.T) { // 1370/AC3
+	in := Have{Build: "b1", Phase: PhaseRemoved, Health: HealthUnknown, Detail: "the declaration no longer holds it"}
+	b, err := EncodeHave(in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out, err := DecodeHave(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	in.V = Version
+	if out != in || out.Phase != "removed" {
+		t.Errorf("round trip: %+v", out)
+	}
+}