Fix ConfigValue initialization in multithreaded environments

[jsiirola]

Fixes # .

Summary/Motivation:

After merging #3394 we started seeing random failures of a multithreading test under pypy. This PR resolves those failures by making the resolution of default Config (Value/List) values more thread safe.

Changes proposed in this PR:

• Add a check to ensure that we only update an uninitialized ConfigValue / ConfigList __class__ once.

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.66%. Comparing base (c463632) to head (12a3e78).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3405      +/-   ##
==========================================
+ Coverage   88.63%   88.66%   +0.03%     
==========================================
  Files         879      879              
  Lines      100363   100365       +2     
==========================================
+ Hits        88955    88992      +37     
+ Misses      11408    11373      -35     

Flag	Coverage Δ
linux	86.12% <100.00%> (+<0.01%)	⬆️
osx	76.11% <100.00%> (+<0.01%)	⬆️
other	86.62% <100.00%> (+<0.01%)	⬆️
win	84.46% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.