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

The prefix handler ran one PeerDown per peer, one after another. When
the sessions of several peers went down together, their walks queued up.
A returning full-table peer's messages stayed held back while the walks
ahead of its own ran, and could reach the limit of 20,000.

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 since the previous one began. The doc of
maxHeldBackMessages says how long a peer's messages are held back. The
limit is now 150,000 messages, about 150 MiB. The status page shows how
many messages each handler holds back.

Model: opus-5-5

internal/routetable/routetable_internal_test.go

@@ -7,11 +7,11 @@ import (
 	"time"
 )
 
-// TestPeerDownLetsReadsIn reads the table's counts again and again while a peer
-// that carries more than two batches of prefixes goes down. PeerDown lets the
-// lock go after every walkBatch prefixes, so a read must get in while the peer
-// still carries some of its prefixes but no longer all.
-func TestPeerDownLetsReadsIn(t *testing.T) {
+// TestPeersDownLetsReadsIn reads the table's counts again and again while a
+// peer that carries more than two batches of prefixes goes down. PeersDown lets
+// the lock go after every walkBatch prefixes, so a read must get in while the
+// peer still carries some of its prefixes but no longer all.
+func TestPeersDownLetsReadsIn(t *testing.T) {
 	t.Parallel()
 
 	const (
@@ -32,8 +32,8 @@ func TestPeerDownLetsReadsIn(t *testing.T) {
 		addr = addr.Next()
 	}
 
-	// Hold a read lock until PeerDown waits for the lock, so that the first
-	// read below starts once PeerDown holds it. TryRLock fails once a writer
+	// Hold a read lock until PeersDown waits for the lock, so that the first
+	// read below starts once PeersDown holds it. TryRLock fails once a writer
 	// waits.
 	table.mu.RLock()
 
@@ -42,7 +42,7 @@ func TestPeerDownLetsReadsIn(t *testing.T) {
 	go func() {
 		defer close(peerDownDone)
 
-		table.PeerDown(peer)
+		table.PeersDown([]string{peer})
 	}()
 
 	for table.mu.TryRLock() {
@@ -60,7 +60,7 @@ func TestPeerDownLetsReadsIn(t *testing.T) {
 
 		select {
 		case <-peerDownDone:
-			t.Fatal("no read got in while PeerDown ran")
+			t.Fatal("no read got in while PeersDown ran")
 		default:
 		}
 	}