Bug 29780: Add Koha::Old::Holds->anonymize

Bug 29780: Add Koha::Old::Holds->anonymize

This patch adds a handy method for anonymizing a Koha::Old::Holds
resultset. It is planned to be used on bug 29526.

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

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>