[SPARK-12077][SQL] change the default plan for single distinct

Use try to match the behavior for single distinct aggregation with Spark 1.5, but that's not scalable, we should be robust by default, have a flag to address performance regression for low cardinality aggregation.

cc yhuai nongli

Author: Davies Liu <davies@databricks.com>

Closes #10075 from davies/agg_15.