PHJ/PA: Plumb level through, change Find() on hash table.

Changes HashCtx to cache evaluated results of the current row.

Change-Id: Id109ce33db8585fdb627ff41a6771dd7004b1951
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4151
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins