Bug: prevent to commit transactions that are too large to be synced

Currently when doing this the WebSocket server throws an error:
```
message: 'Uncaught Error: WebSocket protocol error; protocolError.statusCode = 1009;
  protocolError.description = Message is too large: XXXXX > 1048576'
```

We should avoid getting there, and throw when the user tries to commit a local transaction that's too large.