V2 tentative feature list

[aasseman]

Here's a list of random breaking changes we've been thinking about for a version 2. We are not committing to any of those, and the list may change any time.

• Allocation-less TAP payment sessions. Needed for Horizon / future data services, so this one we are committing to.
• Binary receipts, using RLP encoding
• RAV requests over gRPC instead of JSON-RPC

Note that potential changes span not only this repo, but also probably TAP Subgraph, TAP Contracts, indexer-rs, indexer, or more.