I'm a little against the ```boolean sync``` because we have a bunch of ```ResponseMode```s: 

sync:
```SYNCHRONOUS```
```SYNCHRONOUS_IGNORE_LEAVERS```
```WAIT_FOR_VALID_RESPONSE```

async (depends on the configuration):
```ASYNCHRONOUS```
```ASYNCHRONOUS_WITH_SYNC_MARSHALLING```

other alternative is the when ```boolean sync``` is  ```true``` I set the ```SYNCHRONOUS```, otherwise ```ASYNCHRONOUS```. Then I add the methods ```ignoreLeaver(boolean)``` and ```waitForValidResponses(boolean)```.

but I think this will complicate a little the code. More simpler is ```getRpcOptionsBuilder(ResponseMode)```