Function row_count_demuxer 

async fn row_count_demuxer(
    tx: UnboundedSender<(Path, Receiver<RecordBatch>)>,
    input: SendableRecordBatchStream,
    context: Arc<TaskContext>,
    base_output_path: ListingTableUrl,
    file_extension: String,
    single_file_output: bool,
) -> Result<()>

Dynamically partitions input stream to achieve desired maximum rows per file