p2p, p2p/discover, p2p/discv5: add IP network restriction feature

The p2p packages can now be configured to restrict all communication to
a certain subset of IP networks. This feature is meant to be used for
private networks.

p2p/discv5/net_test.go

@@ -28,7 +28,7 @@ import (
 
 func TestNetwork_Lookup(t *testing.T) {
 	key, _ := crypto.GenerateKey()
-	network, err := newNetwork(lookupTestnet, key.PublicKey, nil, "")
+	network, err := newNetwork(lookupTestnet, key.PublicKey, nil, "", nil)
 	if err != nil {
 		t.Fatal(err)
 	}