Fallback to HTTP POST when WebSocket aren't available

[peaBerberian]

We encounter several devices on which WebSocket are not available / are blocked.

This PR implements a fallback to HTTP POST-based log reporting when the WebSocket connection establishement does not succeed.

The idea for now is that the client send logs to the RxPaired server through HTTP POST every 2/3 seconds.

To divide messages sent in bulk between one another, those are sent as a JSON array in chronological order.

This adds many complexities especially on the server, as now the lifetime of a token might be unclear: is the device still using its token or not? With a WebSocket we just checked the connection state (and did keepalive ping-pong messages), when based on HTTP POST, we now rely on a timeout without any message, for now set to 30 seconds.

For now it's not possible to use sendInstruction when a device relies on HTTP POST. It could theoretically be done through several means including Server-Sent-Events and long polling, but I did not bother for now.

[peaBerberian]

NOTE: I realized that this feature interacts poorly with the "no-token" mode, as each HTTP POST might generate a new token there.

A solution has to be found for that case... (e.g. return generated token as the first http post's response?)

[peaBerberian]

I tried something:

- If using the `/!notoken` route, the server will send back the generated token
  for that connection as a response for that HTTP POST request.

  The client should then use that token for subsequent HTTP POST requests which
  should thus now be on the `/<TOKEN_ID>` URL.