Payload for server owned interfaces missing the timestamp about astarte

[rbino]

I think the actual underlying issue is that we're accepting explicit_timestamp for server-owned interfaces while there's no actual way of sending an explicit timestamp to them from AppEngine API.
explicit_timestamp was designed to provide devices a way to provide an application-specific timestamp to attach to a piece of data, which could possibly be different from the clock timestamp. Does this make sense for server-owned interfaces too? It's open to discussion, but since in the current state there's no actual way to send explicit timestamps for them, I'd suggest to refute explicit_timestamp on server-owned interfaces so we don't provide false hopes to the users. We can always start accepting it if and when we actually have support for it.