feat(internal): thread pool

[jpnurmi]

Introduce a generic thread pool that runs tasks in parallel in a predetermined number of pooled threads and invokes completion callbacks in submission order. The primary use case is the telemetry serialization pool coming in #1946, but it could be used for offloading other heavy tasks, too.

#skip-changelog (internal)

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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Codecov Report

❌ Patch coverage is 82.94574% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.08%. Comparing base (219d934) to head (6ca0dce).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1883      +/-   ##
==========================================
+ Coverage   76.01%   76.08%   +0.07%     
==========================================
  Files          94       94              
  Lines       22183    22441     +258     
  Branches     3937     4017      +80     
==========================================
+ Hits        16863    17075     +212     
- Misses       4436     4468      +32     
- Partials      884      898      +14     

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