network: drop duplicated peer connections by server nonce

On dockerized private network with non-default ext/int ports mapping
there's no way to distinguish connections by underlying connecion
address prior to handshake. Even after handshake, if the peer doesn't
declare announced address/port the same way as it is declared in the
node's local peer record, there's still no other way than server nonce
to distinguish duplicating connections.

For example, on privnet with configuration from #3915, node will
maintain two connection to every seed:
```
// Start the node, wait for some connections to be established.

...

// Try to connect to seed nodes, in particular, with the seed 172.17.0.1:20334:
2025-05-22T10:44:55.779Z	INFO	choosing peer from seeds	{"addr": "172.17.0.1:20334", "requested": 1}
...
2025-05-22T10:44:55.779Z	INFO	dealing address	{"addr": "172.17.0.1:20334"}

...

// New peer is connected (incoming connection), it's the seed node
// 172.17.0.1:20334 in fact, start protocol with this node:
2025-05-22T10:44:55.908Z	INFO	new peer connected	{"peer addr": "172.200.0.254:42654", "fake conn addr": "172.200.0.254:42654", "conn addr": "172.200.0.254:42654", "peerCount": 1}
2025-05-22T10:44:55.909Z	INFO	started protocol	{"addr": "172.200.0.254:42654", "userAgent": "/NEO-GO:0.109.2-pre-16-g953c3580/", "startHeight": 0, "id": 2422991527}

...

// Outgoing seed connection succeeds, there's no way to distinguish this
// connection from incoming one based on any available address:
2025-05-22T10:44:55.994Z	INFO	updating seed address	{"seed": "172.17.0.1:20334", "old": "", "new": "172.17.0.1:20334"}
2025-05-22T10:44:55.994Z	INFO	register connected	{"addr": "172.17.0.1:20334"}
2025-05-22T10:44:55.994Z	INFO	new peer connected	{"peer addr": "172.17.0.1:20334", "fake conn addr": "172.17.0.1:20334", "conn addr": "172.17.0.1:20334", "peerCount": 2}

...

// Start protocol with the same seed based on outgoing connection:
2025-05-22T10:44:55.996Z	INFO	started protocol	{"addr": "172.17.0.1:20334", "userAgent": "/NEO-GO:0.109.2-pre-16-g953c3580/", "startHeight": 0, "id": 2422991527}

```

That's it, addresses can't tell us anything in this case. So we either
live with duplicating connections or filter out connections by server
ID. The latter is not the best option sice an invironment is untrusted
and peer may intentionally fake its ID, but we don't have more reliable
information.

Close #3915.

Signed-off-by: Anna Shaleva <[email protected]>

Author: AnnaShaleva

pkg/network/server.go

@@ -851,15 +851,14 @@ func (s *Server) handleVersionCmd(p Peer, version *payload.Version) error {
 	if s.Net != version.Magic {
 		return errInvalidNetwork
 	}
-	peerAddr := p.PeerAddr().String()
 	s.lock.RLock()
 	for peer := range s.peers {
 		if p == peer {
 			continue
 		}
 		ver := peer.Version()
 		// Already connected, drop this connection.
-		if ver != nil && ver.Nonce == version.Nonce && peer.PeerAddr().String() == peerAddr {
+		if ver != nil && ver.Nonce == version.Nonce {
 			s.lock.RUnlock()
 			return errAlreadyConnected
 		}