Improve writeCheckpoint in MySQL storage implementation

[roger2hk]

#21

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.

Project coverage is 32.92%. Comparing base (46ec9c2) to head (8436b6f).
Report is 5 commits behind head on main.

Files	Patch %	Lines
storage/mysql/mysql.go	0.00%	16 Missing ⚠️
cmd/example-mysql/main.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   35.80%   32.92%   -2.89%     
==========================================
  Files          16       18       +2     
  Lines        1363     1458      +95     
==========================================
- Hits          488      480       -8     
- Misses        801      906     +105     
+ Partials       74       72       -2     

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