Bug 32558: Add ability for background_jobs_worker.pl to process multiple jobs simulta...

Bug 32558: Add ability for background_jobs_worker.pl to process multiple jobs simultaneously up to a limit

Right now background_jobs_worker.pl only processes jobs in serial. It would make sense to handle jobs in parallel up to a user definable limit.

Test Plan:
1) Apply this patch
2) Stop background_jobs_worker.pl
3) Generate some background jobs by editing records, placing holds, etc
4) Watch processes in a new terminal: watch -n 0.1 'ps aux | grep background_jobs_worker.pl'
5) Run background_jobs_worker.pl with parameter -m 3 or some other
   number of max processes
6) Note the multiple forked processes in the ps output

Test notes - also tested the following on KTD:
1. Stop background_jobs_worker.pl
2. Edit /etc/koha/sites/kohadev/koha-conf.xml - set max_processes to 10
3. Generate some background jobs
4. Watch processes in a new terminal: watch -n 0.1 'ps aux | grep background_jobs_worker.pl'
5. Restart all
6. Confirm multiple forked processes in the ps output

Both methods work as expected and generate multiple forked processes
based on the value set for max processes.

Signed-off-by: emlam <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>