peer_set.go
func (ps *PeerSet) List() []*Peer {
	ps.mtx.Lock()
	defer ps.mtx.Unlock()
	return ps.list
}
func (ps *PeerSet) List() []*Peer {
	ps.mtx.Lock()
	defer ps.mtx.Unlock()
	return ps.list
}
// threadsafe list of peers.