Atomic keys update

[jschlyter]

Rewrite code so we only update self._keys once and inside the writer. Thorough review required!

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (633118d) to head (baacdd1).
⚠️ Report is 280 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   77.42%   77.46%   +0.04%     
==========================================
  Files          41       41              
  Lines        4328     4327       -1     
  Branches      838      838              
==========================================
+ Hits         3351     3352       +1     
+ Misses        712      711       -1     
+ Partials      265      264       -1     

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