[AIRFLOW-5870] Allow -1 for infinite pool size

[saguziel]

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Description

Adds the ability to create pools with size=-1, allowing infinite task usage, and allowing the used_slots to return without a db query.

To contextualize this change, we saw spiky DB queries since this has to query all RUNNING task instances, and each task instance that starts running needs to run this query, leading to an n^2 problem.

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Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: add test_infinite_slots

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[KevinYang21]

🙏

[codecov-io]

Codecov Report

Merging #6520 into master will decrease coverage by 0.31%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6520      +/-   ##
==========================================
- Coverage   84.08%   83.76%   -0.32%     
==========================================
  Files         635      635              
  Lines       36849    36851       +2     
==========================================
- Hits        30983    30867     -116     
- Misses       5866     5984     +118

Impacted Files	Coverage Δ
airflow/models/pool.py	97.36% <100%> (+0.14%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/jobs/local_task_job.py	85% <0%> (-5%)	⬇️
airflow/models/taskinstance.py	93.28% <0%> (-0.51%)	⬇️

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[potiuk]

Really nice optimisation. I will also cherry-pick it to 1.10.*. However I will update the description so that it's clear it also optimises DB access.