Batch size is not respected for all indexes

When indexing items via drush, all indexes are indexed in batches of Cron batch size of the first index loaded by search_api.

Steps to reproduce:
- Create two indexes with different values for the Cron batch size setting. E.g. 5 and 10.
- Add enough data to need at least 2 batches for each index
- Run the command drush search-api:index

Actual result:
- All indexes are indexed in steps of the size defined by the Cron batch size setting of the first index loaded.

Expected result:
- Each index is indexed in steps of the size of it's own Cron batch size setting.