[SPARK-3269][SQL] Decreases initial buffer size for row set to prevent OOM

When a large batch size is specified, `SparkSQLOperationManager` OOMs even if the whole result set is much smaller than the batch size.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2171 from liancheng/jdbc-fetch-size and squashes the following commits:

5e1623b [Cheng Lian] Decreases initial buffer size for row set to prevent OOM