Bug 27344: Implement Elastic's update_index_background using Koha::BackgroundJob

Bug 27344: Implement Elastic's update_index_background using Koha::BackgroundJob

This patch adds a background job submodule, UpdateElasticIndex, to deal
with async ES index update (not the deletion).

Using NYTProf (on a checkin):
Without
618ms, executing 35676 statements and 26355 subroutine calls in 266 source files and 83 string evals.

With
521ms, executing 13282 statements and 7979 subroutine calls in 195 source files and 26 string evals.

However there are some problems with this patch:
1. We don't want *all* the index update to be in the background_jobs
tabtle (we could add a filter on the list view)
2. We don't track the "progress" of the job as we are sending all the
records to Elastic. It is okish in my opinion but it must be noted.

Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>