4435: Make update file deletion atomic r=Kerollmops a=irevoire
# Pull Request
## Related issue
Fixes https://github.com/meilisearch/meilisearch/issues/4432
Fixes https://github.com/meilisearch/meilisearch/issues/4438 by adding the logs the user asked
## What does this PR do?
- Adds a bunch of logs to help debug this kind of issue in the future
- Delete the update files AFTER committing the update in the `index-scheduler` (thus, if a restart happens, we are able to re-process the batch successfully)
- Multi-thread the deletion of all update files.
Co-authored-by: Tamo <tamo@meilisearch.com>