config: added support, tests for --externalip config option

Minor change to server.go to add ExternalIPs to
channeldb.LightningNode. Also, added a test that utilizes this
functionality and exercises multiple addresses in NodeAnnouncement.

channeldb/graph_test.go

@@ -23,7 +23,9 @@ import (
 var (
 	testAddr = &net.TCPAddr{IP: (net.IP)([]byte{0xA, 0x0, 0x0, 0x1}),
 		Port: 9000}
-	testAddrs = []net.Addr{testAddr}
+	anotherAddr, _ = net.ResolveTCPAddr("tcp",
+		"[2001:db8:85a3:0:0:8a2e:370:7334]:80")
+	testAddrs = []net.Addr{testAddr, anotherAddr}
 
 	randSource = prand.NewSource(time.Now().Unix())
 	randInts   = prand.New(randSource)