CLPR: Message Queues (POC)

This issue can be closed when the end to end tests are passing application messages through state proofs between two ledgers.

TODO: Consider splitting out back pressure mechanisms and throttles into a different epic

Chunks of Work:

- [ ] Protobuf format for application messages and replies
- [ ] Create test application to generate application messages
- [ ] Create application message handling service
- [ ] Create outgoing message queue
- [ ] Create incoming message queue
- [ ] Create configuration for specifying queue capacities and message throttles.
- [ ] Create gRPC for Message Queue State Proofs
- [ ] Create state proof for throttle configuration to enable back pressure.
- [ ] Create Background queue for handling application messages
- [ ] End to End tests that exercise back pressure through throttles on message sending.

This issue needs to be broken up into sub-issues.