Query transformation of nested queries.

This commit implements nested queries with [NOT] IN, [NOT] EXISTS and
aggregate subquery predicates in Impala. The following cases are
supported:
1. Correlated and uncorrelated [NOT] IN.
2. Correlated [NOT] EXISTS.
3. Correlated and uncorrelated aggregate subqueries.

Change-Id: Ia3f4843c5f07d4e31ef3faedc58a15e623f91a5d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3754
Reviewed-by: Dimitris Tsirogiannis <dtsirogiannis@cloudera.com>
Tested-by: jenkins