Perf regression introduced by SPARK-21113

While running all TPC-DS queries with SF set to 1000, we noticed that Q95 (https://github.com/databricks/spark-sql-perf/blob/master/src/main/resources/tpcds_2_4/q95.sql) has noticeable regression (11%). After looking into it, we found that the regression was introduced by SPARK-21113. Specially, ReadAheadInputStream gets lock congestion. After setting spark.unsafe.sorter.spill.read.ahead.enabled set to false, the regression disappear and the overall performance of all TPC-DS queries has improved.

I am proposing that we set spark.unsafe.sorter.spill.read.ahead.enabled to false by default for Spark 2.3 and re-enable it after addressing the lock congestion issue.