Bug 31328: Make Koha::Item->get_transfer* use Koha::Item::Transfers->filter_by_current

Bug 31328: Make Koha::Item->get_transfer* use Koha::Item::Transfers->filter_by_current

This patch makes the get_transfer and get_transfers methods internally
use the new ->filter_by_current method.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Item.t
=> SUCCESS: Tests pass!
2. Apply this rewriting patch
3. Repeat 1
=> SUCCESS: Tests still pass!
4. Sign off :-D

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