Bug 29843: Add ->anonymize and ->filter_by_anonymizable to Koha::Old::Checkouts

Bug 29843: Add ->anonymize and ->filter_by_anonymizable to Koha::Old::Checkouts

This patch implements high-level methods for:

* Filtering a Koha::Old::Checkouts resultset so only anonymizable rows
  are left
* Anonymizing the rows in a resultset

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Old/Checkouts.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>