Fix otel span being created as a transaction

[giortzisg]

Closes #669. The bug arises from early cleanup logic on the sentry span map. A span can finish before it's child starts, and then the span would be already deleted with the current implementation.

Introducing smarted cleanup, to actually keep track of finished spans on a tree and cleanup only if the whole subtree is finished, to actually correctly link the spans.

[codecov]

Codecov Report

❌ Patch coverage is 90.16393% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.82%. Comparing base (6013529) to head (040e654).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
otel/span_map.go	89.28%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1073      +/-   ##
==========================================
- Coverage   86.85%   86.82%   -0.03%     
==========================================
  Files          55       56       +1     
  Lines        5962     6013      +51     
==========================================
+ Hits         5178     5221      +43     
- Misses        641      647       +6     
- Partials      143      145       +2     

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