[SPARK-9554] [SQL] Enables in-memory partition pruning by default

Author: Cheng Lian <lian@databricks.com>

Closes #7895 from liancheng/spark-9554/enable-in-memory-partition-pruning and squashes the following commits:

67c403e [Cheng Lian] Enables in-memory partition pruning by default

sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala

@@ -200,7 +200,7 @@ private[spark] object SQLConf {
 
   val IN_MEMORY_PARTITION_PRUNING =
     booleanConf("spark.sql.inMemoryColumnarStorage.partitionPruning",
-      defaultValue = Some(false),
+      defaultValue = Some(true),
       doc = "When true, enable partition pruning for in-memory columnar tables.",
       isPublic = false)