Take every down peer out of the table in one walk (closes #225)

The prefix handler ran one PeerDown per peer, one after another, and
stopped taking messages at 20,000 held-back messages. Sessions going
down together queued up their walks, so a returning full-table peer
could reach that limit.

The table's PeersDown now takes several peers out of every prefix in
one walk. One walk runs at a time. The next one takes every peer whose
session went down meanwhile, so a message is held back for about two
walks at most. The limit is now 100,000 messages, about 100 MiB. The
status page shows how many messages each handler holds back.

Model: opus-5-5

internal/server/handlers_internal_test.go

@@ -295,6 +295,43 @@ func TestStatsAnswerCarriesChurnPerMinute(t *testing.T) {
 	}
 }
 
+// heldBackMessages is how many messages heldBackHandler holds back.
+const heldBackMessages = 7
+
+// heldBackHandler is a stream handler that wants no message and holds back
+// heldBackMessages messages.
+type heldBackHandler struct{}
+
+func (heldBackHandler) WantsMessage(string) bool           { return false }
+func (heldBackHandler) HandleMessage(*ristypes.RISMessage) {}
+func (heldBackHandler) QueueCapacity() int                 { return 1 }
+func (heldBackHandler) HeldBackCount() int                 { return heldBackMessages }
+
+// TestStatsAnswerGivesHowManyMessagesAHandlerHoldsBack registers a handler
+// that holds back messages. Its figures in /api/v1/stats, which the status
+// page shows, must give how many.
+func TestStatsAnswerGivesHowManyMessagesAHandlerHoldsBack(t *testing.T) {
+	t.Parallel()
+
+	s := newServerWithTestRoutes(t)
+	s.streamer.RegisterHandler(heldBackHandler{})
+
+	var body struct {
+		Data StatsResponse `json:"data"`
+	}
+
+	err := json.Unmarshal(get(t, s, statsPath).Body.Bytes(), &body)
+	if err != nil {
+		t.Fatalf("decoding the answer: %v", err)
+	}
+
+	stats := body.Data.HandlerStats
+	if len(stats) != 1 || stats[0].HeldBackCount != heldBackMessages {
+		t.Errorf("handler figures %+v, want one handler holding back %d "+
+			"messages", stats, heldBackMessages)
+	}
+}
+
 // fixedIntervalASNFetcher is a WHOIS fetcher that does nothing and reports
 // interval as its wait between fetches.
 type fixedIntervalASNFetcher struct {