pex_reactor.go
func (r *PEXReactor) RequestPEX(p *Peer) {
	p.Send(PexChannel, struct{ PexMessage }{&pexRequestMessage{}})
}
func (r *PEXReactor) RequestPEX(p *Peer) {
	p.Send(PexChannel, struct{ PexMessage }{&pexRequestMessage{}})
}
// RequestPEX asks peer for more addresses.