[SPARK-2554][SQL] Supporting SumDistinct partial aggregation

Adding support to the partial aggregation of SumDistinct

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #3348 from ravipesala/SPARK-2554 and squashes the following commits:

fd28e4d [ravipesala] Fixed review comments
e60e67f [ravipesala] Fixed test cases and made it as nullable
32fe234 [ravipesala] Supporting SumDistinct partial aggregation Conflicts: 	sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregates.scala