CoAP adapter is publish-only, no Observe/subscribe mapping

[dborovcanin]

The CoAP server in server/coap/server.go only handles publish via POST. CoAP Observe (RFC 7641) is not mapped to MQTT subscribe, so CoAP clients have no way to receive messages.

[dborovcanin]

Ideally, we'd rewrite CoAP server completely since once we map its logic to MQTT - we will only need protocol parsing for a complete, 0-dependency implementation.