It's important to address messages to the other peers in the same SQLite transaction as updating our own store (we don't have state-sync yet so this allows us to make sure data is consistent).

Later on, this data will be encrypted and sent through Tunnelbroker (D12132).

Depends on D12129

Make sure messages are persisted (added some mock and read it later).
Tested end-to-end in: D12133