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/streamer/streamer.go

@@ -351,6 +351,10 @@ type HandlerStats struct {
 	AvgProcessTime     time.Duration
 	MinProcessTime     time.Duration
 	MaxProcessTime     time.Duration
+	// HeldBackCount is how many messages the handler holds back to apply
+	// later: what its HeldBackCount method returns, or 0 for a handler
+	// without one.
+	HeldBackCount int
 }
 
 // GetHandlerStats returns a snapshot of performance statistics for all registered
@@ -392,6 +396,11 @@ func (s *Streamer) GetHandlerStats() []HandlerStats {
 
 		info.metrics.mu.Unlock()
 
+		holder, holdsBack := info.handler.(interface{ HeldBackCount() int })
+		if holdsBack {
+			hs.HeldBackCount = holder.HeldBackCount()
+		}
+
 		stats = append(stats, hs)
 	}