[SPARK-14031][SQL] speedup CSV writer

## What changes were proposed in this pull request?

Currently, we create an CSVWriter for every row, it's very expensive and memory hungry, took about 15 seconds to write out 1 mm rows (two columns).

This PR will write the rows in batch mode, create a CSVWriter for every 1k rows, which could write out 1 mm rows in about 1 seconds (15X faster).

## How was this patch tested?

Manually benchmark it.

Author: Davies Liu <davies@databricks.com>

Closes #13229 from davies/csv_writer.