Add support for uncorrelated EXISTS subqueries

This commit adds support for uncorrelated EXISTS subqueries in Impala.
Uncorrelated EXISTS subqueries are rewritten using a CROSS JOIN.
Uncorrelated NOT EXISTS subqueries are not supported.

Change-Id: I0003dcdc0fa5cc99931b9a9f4deddbcd42572490
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4140
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins