Bug 22456: Add cancellation request methods to Koha::Hold

Bug 22456: Add cancellation request methods to Koha::Hold

This patch adds the following helper methods for dealing with hold
cancellation requests to the Koha::Hold class:

 * cancellation_requests
 * add_cancellation_request
 * cancellation_requestable_from_opac
 * cancellation_requested_by_owner

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

Sponsored-by: Montgomery County Public Libraries
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>