Bug 8918: Fix reserve priority in ILS-DI

Bug 8918: Fix reserve priority in ILS-DI

The priority of new hold requests was not calculated when using ILS-DI.

A new routine is added, C4::Reserves::CalculatePriority(), to calculate
the priority prior to placing a request.

A separate bug report, 11640, covers the changes in reserves to
use this new routine more generally.

This patch does therefore only affect ILS-DI.

Note: ILS-DI already allows you to generate multiple holds on a biblio or
item for the same patron. This patch does not change that behavior.

Test plan:
[1] Place multiple holds using ILS-DI HoldTitle service:
    /cgi-bin/koha/ilsdi.pl?service=HoldTitle&patron_id=BORROWERNUMBER&bib_id=BIBLIONUMBER&request_location=test
    Check the priority.
[2] Do the same using HoldItem service:
    /cgi-bin/koha/ilsdi.pl?service=HoldItem&patron_id=BORROWERNUMBER&bib_id=BIBLIONUMBER&item_id=ITEMNUMBER
    Check the priority again.
[3] Use a biblio with multiple items. Place item level holds on both.
    Check in one of these items in another branch. Confirm transfer.
    Check in the other item in the original branch. Confirm hold.
    Now you have a waiting and a transit hold.
    Test HoldTitle and HoldItem service again a few times.
[4] Enable AllowHoldDateInFuture and add a future hold.
    Now test HoldTitle and HoldItem again and check if these holds are
    inserted before the future hold (lower priority).

January 29, 2014: Rebased this patch and amended it to make a distinction
between fixing the ILS-DI bug and using the new routine.
Updated commit message and test plan (marcelr).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 3e344d7e0718e5a8ac810ca5235f68c1e6d3f370)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>