Bug 29507: Speed up auto renew cronjob via parallel processing

Bug 29507: Speed up auto renew cronjob via parallel processing

The cron can take a very long time to run on systems with many issues.
For example, a partner with ~250k auto_renew issues is taking about 9 hours to run.

If we run that same number of issues in 5 parallel chunks
( splitting the number of issues as evenly as possible ), it could take under 2 hours.

Test Plan:
1) Generate a number of issues marked for auto_renew
2) Run the automatic_renewals.pl, use the `time` utility to track how much time it took to run
3) Set parallel_loops to 10 in auto_renew_cronjob section of config in koha-conf
4) Repeat step 2, note the improvement in speed
5) Experiment with other values

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>