Adapted Node.js server to new message structure and protocol

* Messages sent to the signaling server directly ('to' field set to
  'signaling-server') are just ignored. Those messages are ACKs from 'denied'
  signaling messages.
* Messages that have a proper 'to' field set which is not '*' are just
  forwarded without further processing.
* The server now ACKs ROUTE messages before sending the 'denied' signaling
  message.