Bug 7846 - get_batch_summary massive speedup

Bug 7846 - get_batch_summary massive speedup

current code is using DISTINCT and another SQL query which can be replaced with GROUP BY
for massive speedup. In our case, generating Manage Batches screen DBI time decreased
from 24.762 s to 0.147 s

Aside from correct usage of relational database, this change also cleans up code nicely.

This change removed semi-columns from SQL query which broke Manage Patron batches.

Test scenario:

1. open Manage Batches screen and take note of time needed to generate it
2. apply this patch
3. reload page and check page genration time

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>