Planning and grouping of multiple analytic exprs from a select block.

This patch adds support for:
- Planning of multiple analytic exprs from a select block
- Simple grouping of analytic exprs by partition/order/window
  to reduce data exchanges and sorts

Change-Id: Ie2162558b2bc2e6218c30e694393e85cbf3251ff
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4120
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4168