add MaxPeerNumber() option to p2p

[dustinxie]

What would you like to be added:
when p2p creates a Host instance, it passed down couple of options:

func (p *agent) Start(ctx context.Context) error {
	ready := make(chan interface{})
	p2p.SetLogger(log.L())
	opts := []p2p.Option{
		p2p.HostName(p.cfg.Host),
		p2p.Port(p.cfg.Port),
		p2p.Gossip(),
		p2p.SecureIO(),
		p2p.MasterKey(p.cfg.MasterKey),
		p2p.PrivateNetworkPSK(p.cfg.PrivateNetworkPSK),
		p2p.DHTProtocolID(p.chainID),
		p2p.DHTGroupID(p.chainID),
	}

add another option MaxPeerNumber(). A node is expected to maintain connection to (80% of this number) many peers to be considered healthy

Why is this needed:
to work with ongoing p2p improvement