Fix more concurrency issues

[0xnm]

What does this PR do?

A bit more of concurrency issues fixes. The main change is shifting from getOrPut call, which is not atomic.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.30%. Comparing base (4389648) to head (d6b56f4).
⚠️ Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
...m/internal/metric/slowframes/SlowFramesListener.kt	77.78%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3197      +/-   ##
===========================================
- Coverage    71.36%   71.30%   -0.06%     
===========================================
  Files          928      928              
  Lines        34444    34447       +3     
  Branches      5814     5815       +1     
===========================================
- Hits         24579    24562      -17     
  Misses        8239     8239              
- Partials      1626     1646      +20     

Files with missing lines	Coverage Δ
...oid/rum/internal/vitals/AggregatingVitalMonitor.kt	96.30% <100.00%> (ø)
...m/internal/metric/slowframes/SlowFramesListener.kt	94.64% <77.78%> (-3.47%)	⬇️

... and 36 files with indirect coverage changes

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