[VL] Slow scan with Spark runtime filter when split-preloading is turned on

[zhztheplayer]

In some cases when runtime filter is added, spark.gluten.sql.columnar.backend.velox.IOThreads=0 (which turns Velox's preload / prefetch off) could gain better performance than the default settings (spark.gluten.sql.columnar.backend.velox.IOThreads=<task slot num>).

Caused by facebookincubator/velox#10173

To temporarily bypass the issue if found, just set spark.gluten.sql.columnar.backend.velox.IOThreads=0.