[2.3] - Add schedule backlog to indexer:status

[convenient]

• PR for 2.1 = [2.1] - Add command to view mview state and queue #12050
• PR for 2.2 = [2.2] - Add command to view mview state and queue #12122
• PR for 2.3 = [2.3] - Add schedule backlog to indexer:status #12592

Description

Update indexer:status to output information about the schedule mview backlog

Manual testing scenarios

1. Enable an indexer php bin/magento indexer:set-mode schedule catalog_product_price
2. Trigger an action on the product price that will update the _cl table
3. Run php bin/magento indexer:status to see the backlog increase
4. magerun2 sys:cron:run indexer_update_all_views will trigger the mview processes
5. See the backlog decrease

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)