Bug 29525: Add Koha::Old::Holds->filter_by_anonymizable

Bug 29525: Add Koha::Old::Holds->filter_by_anonymizable

This patch adds the 'filter_by_anonymizable' method, and tests for it. A
new DBIC relationship is added as well to the OldReserve schema file.

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

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>