Fixed circular reference in HTTPConnection

[francis-clairicia]

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

Resolves #746

❓ What is the current behavior? (You can also link to an open issue here)

The UNIX socket server's caching system for credentials involves a circular reference to HTTPConnection, even for an INET server.

❓ What is the new behavior (if this is a feature change)?

The cache system is still in place, but now uses private attributes.

📋 Other information:

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• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (ac9b6e5) to head (ff66b04).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #755      +/-   ##
==========================================
- Coverage   84.01%   83.99%   -0.03%     
==========================================
  Files          28       28              
  Lines        4235     4242       +7     
==========================================
+ Hits         3558     3563       +5     
- Misses        677      679       +2