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 two walks at
most. Each walk takes about 2.6 seconds for each full-table peer it
takes out. The limit is now 150,000 messages, about 150 MiB: enough
when four full-table peers go down together. The status page shows how
many messages each handler holds back.

Model: opus-5-5

internal/routetable/reads_test.go

@@ -227,7 +227,7 @@ func TestCounts(t *testing.T) {
 	})
 
 	// peerC was the last peer of the IPv6 prefix. It keeps its number.
-	table.PeerDown(peerC)
+	table.PeersDown([]string{peerC})
 	checkCounts(t, table, routetable.Counts{
 		Pairs:                2,
 		Peers:                3,