fix: ensure atomicity of the fetch and reschedule operation

[reugn]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.08108% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 95.30%. Comparing base (50c609c) to head (f7cb93b).

Files	Patch %	Lines
quartz/scheduler.go	81.08%	4 Missing and 3 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   94.63%   95.30%   +0.67%     
==========================================
  Files          19       19              
  Lines         951      959       +8     
==========================================
+ Hits          900      914      +14     
+ Misses         32       27       -5     
+ Partials       19       18       -1     

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