fix: avoid deadlocking singleflighted reqs

[miparnisari]

Description

Potential fix for #2788

TO DO: discuss with @josephschorr. I want to overhaul the singleflight code (specifically singleflight.go, optimized.go, and context.go).

• move the code of optimized.go into singleflight.go
• move CRDB's featureGroup into singleflight.go

Testing

docker-compose -f docker-compose.deadlock.yaml up -d
sh reproduce_deadlock.sh

reproduce_deadlock.sh
docker-compose.deadlock.yaml

References

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.57%. Comparing base (97811cf) to head (d6f53ed).

Files with missing lines	Patch %	Lines
internal/datastore/proxy/singleflight.go	89.48%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3048      +/-   ##
==========================================
+ Coverage   75.57%   75.57%   +0.01%     
==========================================
  Files         489      489              
  Lines       59837    59864      +27     
==========================================
+ Hits        45213    45234      +21     
- Misses      11332    11338       +6     
  Partials     3292     3292              

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