Use CBOR for device messages (SenML-over-CBOR)

[drasko]

Currently we use JSON SenML for device messaging. Using JSON for telemetry messages for embedded devices is heavy and unsuitable for several reasons:

• JSON encoding is not simple for embedded devices. As a matter of fact, I found only one suitable JSON parser for these kind of devices that have no OS (so no system calls) and use their own C stdlib: https://zserge.com/jsmn.html
• Binary messages are more compact and this have 2 consequences:
  • Battery is preserved (as most of the battery is spent on radio Tx bursts)
  • Bandwidth is cheaper - especially important when you pay cellular monthly plans for CAT-M/NB-IoT

Both of these points (battery and bandwidth price) are crucial.

SenML supports CBOR encoding by standard.

Additionally, and related to #367, we can use COSE for security.

[evarga]

I agree that this is a good choice, since support of CBOR is part of the SenML specification. Moreover, CBOR is also a proposed standard under RFC 7049.