Use sync/atomic instead of Uber's implementation

[torwig]

Thanks to @aleksraiden for the hint.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.

Project coverage is 47.40%. Comparing base (6c56470) to head (dee33dd).
Report is 32 commits behind head on unstable.

Files with missing lines	Patch %	Lines
store/cluster.go	87.09%	3 Missing and 1 partial ⚠️
controller/controller.go	0.00%	0 Missing and 2 partials ⚠️
store/engine/raft/node.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #256      +/-   ##
============================================
+ Coverage     43.38%   47.40%   +4.01%     
============================================
  Files            37       43       +6     
  Lines          2971     3932     +961     
============================================
+ Hits           1289     1864     +575     
- Misses         1544     1874     +330     
- Partials        138      194      +56     

Flag	Coverage Δ
unittests	47.40% <80.55%> (+4.01%)	⬆️

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[jihuayu]

LGTM, thank you!