Bug 30360: Add helper methods to Koha::BackgroundJobs

Bug 30360: Add helper methods to Koha::BackgroundJobs

This patch adds some helper methods to ease writing background jobs, and
also making it more consistent/solid.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/BackgroundJob.t
=> SUCCESS: Tests pass. The methods have the expected behaviour.
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>