Fix issue #3163

[Haeki]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests ran successful (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

Current code:

If the node is running while next(node) is called, waiting.find_node_index(node) will throw an exception because the running node is not in this waiting list.
The indented behavior is a direct return from the next function.

My changes

I changed to code so that it uses the find_index() function instead, because if the node is not found there, it will simply return nil.
I also switched return unless waiting.find_index(node) to return if running.find_index(node). The running list is just the inverse of the waiting list, but it should be much shorter.
To be able to call find_index() on the running/waiting list, the function had to be moved from the private scope to the public scope.

I ran the tests and rubocop without any problems. I also tested it directly in our oxidized installation and it fixed the Web API /next returns status 500 if node is waiting to poll issue.

Closes issue #3163

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.30%. Comparing base (950d4e1) to head (1e08025).
Report is 14 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3250      +/-   ##
==========================================
- Coverage   63.53%   62.30%   -1.24%     
==========================================
  Files          37       39       +2     
  Lines        1862     1955      +93     
==========================================
+ Hits         1183     1218      +35     
- Misses        679      737      +58     

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