Partitioned aggregation.

Change-Id: Ie3acfa04e359194b5b40011107910293f1e5609e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3788
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3948