fix: Prevent multiple SessionCache instances from being created. (#2180)

[tippmar-nr]

Thank you for submitting a pull request. Please review our contributing guidelines and code of conduct.

Description

Prevents an edge case where multiple SessionCache instances could get created if StackExchange.Redis.ConnectionMultiplexer.CreateMultiplexer() is called multiple times.

Improves resiliency in SessionCache.Cleanup() and adds logging.

Fixes #2180

Author Checklist

• Unit tests, Integration tests, and Unbounded tests completed
• Performance testing completed with satisfactory results (if required)

Reviewer Checklist

• Perform code review
• Pull request was adequately tested (new/existing tests, performance tests)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (445f7c2) 81.05% compared to head (a8e7d0a) 81.02%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2186      +/-   ##
==========================================
- Coverage   81.05%   81.02%   -0.04%     
==========================================
  Files         396      396              
  Lines       24723    24719       -4     
  Branches     2997     2997              
==========================================
- Hits        20040    20029      -11     
- Misses       3895     3903       +8     
+ Partials      788      787       -1     

Files	Coverage Δ
src/Agent/NewRelic/Agent/Core/Agent.cs	71.05% <ø> (-0.19%)	⬇️

... and 2 files with indirect coverage changes

[chynesNR]

I don't think this will hurt anything, but I also suspect this won't address the issue we're seeing.