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/console/runtimes_test.go

@@ -23,18 +23,22 @@ func slowRuntime(t *testing.T, conn *bus.Conn, node string, lag time.Duration, e
 		t.Fatal(err)
 	}
 	t.Cleanup(stop)
-	stop, err = conn.Raw("$SRV.INFO", func(string, []byte) []byte {
+	// Asked of every service, and of this instance by name and id — as discovery asks it (issue 554).
+	info := func(string, []byte) []byte {
 		if lag < 0 {
 			return nil
 		}
 		time.Sleep(lag)
 		body, _ := json.Marshal(announce.Info(announce.Service{Name: id.Name, ID: id.ID, Metadata: id.Metadata}, endpoints))
 		return body
-	})
-	if err != nil {
-		t.Fatal(err)
 	}
-	t.Cleanup(stop)
+	for _, subject := range []string{"$SRV.INFO", "$SRV.INFO." + id.Name + "." + id.ID} {
+		stop, err := conn.Raw(subject, info)
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Cleanup(stop)
+	}
 	conn.Flush()
 }
 
@@ -175,7 +179,7 @@ func TestARuntimeHeardBeforeAndNotNowIsNamed(t *testing.T) {
 	s.remember(before)
 	now := &index{Modules: map[string]*moduleInfo{"nextcloud-client": {Module: "nextcloud-client", On: []string{"desktop"},
 		Tools: []Tool{{Module: "nextcloud-client", Name: "nextcloud_check"}}}}, Recorded: map[string][]string{},
-		Discovery: announce.Discovery{At: time.Now().UTC()}}
+		Discovery: announce.Discovery{At: time.Now().UTC(), Pinged: 1}} // desktop's runtime answered
 	s.remember(now)
 	if len(now.Unheard) != 1 || now.Unheard[0].Machine != "laptop" {
 		t.Fatalf("unheard: %+v", now.Unheard)
@@ -190,7 +194,7 @@ func TestARuntimeHeardBeforeAndNotNowIsNamed(t *testing.T) {
 		t.Errorf("%v", err)
 	}
 	// Nobody unheard: the plain answers stand.
-	quiet := &index{Modules: now.Modules, Recorded: map[string][]string{}}
+	quiet := &index{Modules: now.Modules, Recorded: map[string][]string{}, Discovery: now.Discovery}
 	if _, err := resolve(quiet, "laptop/nextcloud-client.nextcloud_check"); err == nil || !strings.Contains(err.Error(), "does not run on laptop; it runs on desktop") {
 		t.Errorf("%v", err)
 	}