Function write_rows_single_ts 

pub async fn write_rows_single_ts(
    client: &PersistClient,
    shard: ShardId,
    desc: &RelationDesc,
    rows: &[Row],
    ts: Timestamp,
) -> Result<()>

Writes rows to shard at ts, advancing upper to ts+1. All rows are inserted with diff +1. Returns once the append succeeds.