Bug 26457: (QA follow-up) Switch to PK index in UPDATE on issues

Bug 26457: (QA follow-up) Switch to PK index in UPDATE on issues

The deadlock reports tell us that multiple transactions are
waiting for a X lock on a record but using a secondary index
on borrowernumber and itemnumber. Since we have the issue_id
at hand already, we should use that and benefit from the clustered
index (on PK) instead of using a secondary index.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>