Fix incorrect slots stats (#15399)

[wongelz]

Fixes the incorrect number of queued and running slots, and therefore, open slots, when there exist task instances that occupy > 1 pool_slots. This was causing the scheduler to over-commit to a given pool, and a subsequent state where no further tasks can be scheduled because slots cannot be freed.

closes: #15399

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[boring-cyborg]

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[kaxil]

Thanks @wongelz 🚀