Bug 22690: Refactor merging of records to improve performance (Elasticsearch)

Bug 22690: Refactor merging of records to improve performance (Elasticsearch)

This patch allows merging of records with many items without the web server timing out.

Test plan:

Without the patch:

- Create 2 records (one with e.g. 1000 items).
- Do a cataloguing search that displays both records, select them and click "Merge selected".
- Choose the record with many items as the one to be eliminated.
- Start the merging.
- After a while the web server should give you a timeout error (the merging process may still continue)

With the patch:
- Do the same as above
- This time verify that the records are merged without timeout
- Create a new biblio with an item
- Add with the item:
  * acquisition order
  * hold (reserve)
- Merge the biblio to another one
- Verify that the item and its related data was moved
- Verify that tests pass:
  prove -v t/db_dependent/Koha/Biblio.t
  prove -v t/db_dependent/Koha/Item.t
  prove -v t/db_dependent/Koha/SearchEngine/Indexer.t

Signed-off-by: Michal Denar <black23@gmail.com>
Rebased-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>