peer.go
func (l *PeerList) Len() int {
	l.RLock()
	defer l.RUnlock()
	return l.peerHeap.Len()
}
func (l *PeerList) Len() int {
	l.RLock()
	defer l.RUnlock()
	return l.peerHeap.Len()
}
// Len returns the length of the PeerList.