Bug 24440: Add ->current_holds to Koha::Acquisition::Order

Bug 24440: Add ->current_holds to Koha::Acquisition::Order

This patch introduces a method to fetch the current holds associated with the
items linked to an order line. It basically implements what's done in
parcel.pl, but fully tested and suitable for using on the API.

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Order.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>