Bug 27783: Add --job-type to background_jobs_worker.pl

Bug 27783: Add --job-type to background_jobs_worker.pl

This will let sysop adjust the number of workers and how they want to
manage them.
For instance one could want to have one worker for ES indexation and
another worker for other jobs, to prevent ES index to be stuck behind
bigger batch process.

Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>