Rewrite NOT IN subqueries with a null-aware anti-join.

This commit fixes the issue (IMPALA-1215) where NOT IN subqueries return
wrong results in the presence of NULL values. The null-matching equality
operator is introduced in the front-end and the NOT IN subqueries are
rewritten using the null-aware anti-join operator.

Change-Id: I5a323357025d77c2143db86e1057999ec8a371c0
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4391
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins