fix(circuit breaker): protect access to lastFailureAt by atomic operations to avoid race conditions

[liuzengh]

• Remove unnecessary nil checks for the receiver. it is the caller's responsibility to ensure that circuit breaker is not nil.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (66256ef) to head (90c5a7d).

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #1019      +/-   ##
==========================================
- Coverage   99.82%   99.82%   -0.01%     
==========================================
  Files          18       18              
  Lines        3919     3915       -4     
==========================================
- Hits         3912     3908       -4     
  Misses          5        5              
  Partials        2        2              

Flag	Coverage Δ
unittests	99.82% <100.00%> (-0.01%)	⬇️

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