Private gateways are unable to authorise public gateways to send them cargoes

[gnarea]

Which means that, for example, the Android Gateway will refuse cargo from the Relaynet-Internet Gateway, even though cargo can still be sent in the opposite direction.

The solution should be to issue short-lived, single-use certificates to public gateways, and attach such certificates to the CCA. Public gateways won't have to store them, as they can simply extract it from the CCA and sign outgoing cargo.

In other words, we'll be using delivery authorisations like the ones we currently use to deliver parcels.