Keep what discovery heard when a round hears nothing, and fetch only changed announcements (issue 554)

A round in which no PING reached the laptop's console within the 750 ms window
was taken as the mesh's answer that nothing is there, and replaced the index:
every seat and module call was refused for five seconds at a time. Discovery
now waits up to its patience for a first PING, a round that heard nothing is
not kept, a runtime not heard this round is resolved from what it announced
last (said with its age), and a round that misses runtimes is logged once.
A PING carries a digest of what the instance serves, so an unchanged
announcement is not pulled again every round (some 880 kB to the laptop).

node-tools/internal/announce/gather_test.go

@@ -33,18 +33,22 @@ func lagging(t *testing.T, conn *bus.Conn, s Service, lag time.Duration, endpoin
 		t.Fatal(err)
 	}
 	t.Cleanup(stop)
-	stop, err = conn.Raw("$SRV.INFO", func(string, []byte) []byte {
+	// Asked of every service, and of this instance by name and id — as discovery asks it (issue 554).
+	info := func(string, []byte) []byte {
 		if lag < 0 {
 			return nil
 		}
 		time.Sleep(lag)
 		body, _ := json.Marshal(Info(s, endpoints))
 		return body
-	})
-	if err != nil {
-		t.Fatal(err)
 	}
-	t.Cleanup(stop)
+	for _, subject := range []string{"$SRV.INFO", "$SRV.INFO." + s.Name + "." + s.ID} {
+		stop, err := conn.Raw(subject, info)
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Cleanup(stop)
+	}
 	conn.Flush()
 }
 
@@ -73,7 +77,7 @@ func TestDiscoveryWaitsForEveryRuntimeThatSaidItIsThere(t *testing.T) {
 
 	asker := connect(t, "console", "desk")
 	start := time.Now()
-	d, err := Gather(asker)
+	d, err := Gather(asker, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -109,7 +113,7 @@ func TestDiscoveryDoesNotWaitWhenEveryoneAnswered(t *testing.T) {
 	t.Cleanup(stop)
 	fast.Flush()
 	start := time.Now()
-	d, err := Gather(connect(t, "console", "desk"))
+	d, err := Gather(connect(t, "console", "desk"), nil)
 	if err != nil {
 		t.Fatal(err)
 	}