Libxmtp should be able to speak to the new nodes for all core functionality.

Includes:

• New API client talking to xmtpd
• Including dependencies on published messages
• Topological sort of returned messages
• Handling new originator ID and sequence_id
• Support for reliable streams
• Basically everything in #1637

Does not include:

• Client signatures of Authenticated Data
• Misbehavior reports
• Payer Service in the client