Draft protocol to allow endpoints to stream data via a public gateway

[gnarea]

To support applications like VoIP, for example.

The public gateway will effectively act as a proxy. All the usual authentication and authorisation from Awala will apply, so only pre-authorised endpoints will be able to initiate the stream. Likewise, the stream will be end-to-end encrypted, although this would have to be implemented in a totally different way using stream ciphers (we can't use parcels or CMS EnvelopedData).

Unlike #47, peers won't be able to see each other's IP addresses.