IMPALA-1247: In a 2-phase agg the 1st phase should output its intermediate tuple.

Change-Id: I8f7ba0551099b6cf524baf6bd6f848d02896418d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4378
Tested-by: jenkins
Reviewed-by: Alex Behm <alex.behm@cloudera.com>