HIVE-3728. make optimizing multi-group by configurable. (njain via kevinwilfong)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1424292 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'mthive-0.10' of github.com:MTDATA/hive into mthive-0.10