Fix spilling with aggregated exprs with string slots.

This patch also does some clean up on agg-fn-evaluator to support this case.

Change-Id: If7e5c8663c7d371b2666acaab2966a2cd7bdccf9
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4256
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>