[Issue #322] Change the way executors are calculated to fix indefinite pending queue

[mtn-boblloyd]

Change the way the executors are calculated by using the LoadStatisticsSnapshot rather than the strategyState object in Jenkins. This way, the number of executors is directly calculated by the available and connecting executors minus the busy executors, rather than considering ambiguous pending, and additionally planned executors, that can create availability that doesn't actually exist.

Testing done

Running this locally in our Jenkins instance for the last month without issue. No further testing was done.

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[pdk27]

@mtn-boblloyd Thanks for this PR, appreciate it.

A release with a fix for computations was just published. Here is the PR fix if you are interested in some context / details.
Here is the discussion thats open for further feedback on the release - #372.

Closing this PR for now. If you see unaddressed concerns, please leave a comment in the discussion or (re)open the related issue or open a PR.