LabLink 2.3.1

🐛 Fixed

• A bench that was already connected could not be reconnected. After a
  server update the scope and both supplies refused to connect while the
  server was still holding all three open and serving readings from them.
  connect always opened the resource again, so pressing Connect asked
  libusb to claim an interface the server itself was holding; it answered
  [Errno 16] Resource busy and the API returned 500 -- a refusal that reads
  like broken hardware and is really the server colliding with itself. An
  instrument that is already open is returned as it stands, and an entry that
  is present but no longer connected is dropped first, since there a real
  open is wanted.
• A websocket stream retried a command the instrument does not have. A
  scope asked for get_readings logged "Unknown command" twice a second,
  indefinitely. Every pass sends the same command, so what fails once fails
  always; the stream stops now.
• A timestamp in a broadcast disconnected the client. send_json uses a
  plain json.dumps, which cannot encode the datetime in every readings
  payload, and the failure was raised inside the per-connection loop -- so a
  serialisation bug was read as the client hanging up and the websocket was
  dropped. The message is encoded once, before any connection is touched.