Clarify interface with data sync layer

[oskarth]

We want to make sure the mixnet layer and above data sync layers interface properly, and that we are mindful of any friction that might result.

See https://discuss.status.im/t/data-sync-next-steps-and-considerations/1056 for next steps and considerations, as well as https://github.com/status-im/status-research/tree/master/data_sync for current PoC and spec.

Additionally:

• Explore Bramble vs TreeKEM and Matrix
• Sacrifice some metadata to semi-trusted nodes

Acceptance criteria

It is clear what the above data sync layer will look like, and what requirements it has of the underlying layer.