Support SSDP in LAN

[garyyu]

Currently we limit the connection in the same NAT address: only one connection is allowed .

	/// Checks whether there's any reason we don't want to accept an incoming peer
	/// connection. There can be a few of them:
	/// 1. Accepting the peer connection would exceed the configured maximum allowed
	/// inbound peer count. Note that seed nodes may wish to increase the default
	/// value for PEER_LISTENER_BUFFER_COUNT to help with network bootstrapping.
	/// A default buffer of 8 peers is allowed to help with network growth.
	/// 2. The peer has been previously banned and the ban period hasn't
	/// expired yet.
	/// 3. We're already connected to a peer at the same IP. While there are
	/// many reasons multiple peers can legitimately share identical IP
	/// addresses (NAT), network distribution is improved if they choose
	/// different sets of peers themselves. In addition, it prevent potential
	/// duplicate connections, malicious or not.
	fn check_undesirable(&self, stream: &TcpStream) -> bool {

At this moment, I don't want to change this behaviour, but In case someone have multiple node servers running in the same NAT, these servers can't connect to each other because above limitation.

A simple solution to solve this is to use the SSDP (Simple Service Discovery Protocol), allowing the node servers discovery each other in same LAN and connect each other.

A Rust SSDP crate can be used for this: https://github.com/GGist/ssdp-rs

Note: To avoid the LAN ip address publishing to peers network, the GetPeerAddrs protocol response should skip all these LAN ip addresses.