====== maintenance ======
**S → C {type:"maintenance", mode:..., msg:...}**

maintenance – maintenance message from server. IXNX server operator can from configuration GUI send maintenance messages to all connected clients.

arguments:
  * mode - maintenance mode:
      * "start": start of maintenance mode. Service may become limited from now on.
      * "end": end of maintenance mode. Normal service level again.
  * msg - free text message