Fix lru cache copying

[ffelixg]

This addresses the lru cache copy function by walking the circularly linked list and creating new cache/root attributes instead of using (deep-)copy on the source.

Fixes #3852

[antonpirker]

This looks good to me. Because it is some tricky code, I will ask for some other engineers to review it before I will merge it.

[cmanallen]

@ffelixg Thanks for your interest in fixing this bug! I left some review comments. You can also review my PR here to see how I implemented it: https://github.com/getsentry/sentry-python/pull/3882/files. Obviously I'm biased in favor of my solution to the problem but you're welcome to let me know where I've made short comings!

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.89%. Comparing base (8ced660) to head (f98f6a9).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/_lru_cache.py	93.10%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3883      +/-   ##
==========================================
- Coverage   79.91%   79.89%   -0.02%     
==========================================
  Files         139      139              
  Lines       15458    15429      -29     
  Branches     2625     2624       -1     
==========================================
- Hits        12353    12327      -26     
+ Misses       2232     2228       -4     
- Partials      873      874       +1     

Files with missing lines	Coverage Δ
sentry_sdk/_lru_cache.py	94.87% <93.10%> (-5.13%)	⬇️

... and 3 files with indirect coverage changes

[antonpirker]

After a quick discussion here #3852 we will settle on a simpler lru cache implementation.

This looks fine now and has all the tests, so it is good to be merged.

Review was for an old implementation.