Conditional requests
GET /device/v1/applications and GET /device/v1/parameters
return an ETag header. Send it back in If-None-Match on the next
request. If nothing has changed, the response is 304 Not Modified with no
body.
- Treat the value as opaque.
- Store it only after the terminal has acted on the response.
- Discard it when the terminal is activated again.
- Heartbeat responses carry no
ETag, so take the first one from aGET.