Bug 23624: (QA follow-up) Optimize even more

Bug 23624: (QA follow-up) Optimize even more

This patch makes counting the results have no memory footprint by
leveraging on the DB to count the rows.

To test:
- Without this path, run:
  $ kshell
 k$ prove t/db_dependent/Reports/Guided.t
=> SUCCESS: Tests pass
- Apply this patch
- Run:
 k$ prove t/db_dependent/Reports/Guided.t
=> SUCCESS: Tests still pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>