The way I see it, RpcOptionsBuilder is meant to be mutable, so we should allow the user to change anything after construction. Plus, if we keep the responseMode method, so we can do stuff like this:

    syncIgnoreLeavers = new RpcOptionsBuilder(syncTemplate).responseMode(SYNCHRONOUS_IGNORE_LEAVERS)

I'd rather have a `boolean sync` parameter on `getRpcOptionsBuilder` though, because it's very similar in functionality to `getDefaultRpcOptions`, and the user should be able to switch between the two without thinking too much.