Bug 35086: Also split chunks when indexing from background job

Bug 35086: Also split chunks when indexing from background job

The es background indexer is designed to combine background jobs when started based on the 'batch_size' option.

While this is helpful for combining individual updates, it can be problematic when there are several large batch modifications, or when worker has stopped and is restarted.

This patch uses the same logic as in the indexer to split the chunks that are sent directly for indexing.

To test:
1 - Follow test plan on previous patch
2 - Confirm items are correctly indexed and jobs marked

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>