load data: load parquet to online, unable to ensure correctness

load data can't ensure correctness: insert more rows to online than parquet file

Steps:
1. load data from parquet ( 600 million rows, 3.5 G parquet file in disk, 300 parquet files total )
2. with spark local, driver memory: 64G, max task in parallel: 8
3. spark will split job into 50 tasks, and 8 tasks one time
4. tasks might fail due to `IOException`, rpc to tablet server timeout of 20s
5. spark sees to retry the whole job (50 tasks), result in insert data from parquet double times