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/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)
 	}