Ensure to remove the oldest segment when flushing

[nakabonne]

No description provided.

[codecov-commenter]

Codecov Report

Merging #34 (90f389b) into main (e7ef1a5) will decrease coverage by 0.41%.
The diff coverage is 64.11%.

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
- Coverage   62.74%   62.34%   -0.40%     
==========================================
  Files          19       19              
  Lines        1111     1123      +12     
==========================================
+ Hits          697      700       +3     
- Misses        288      292       +4     
- Partials      126      131       +5     

Impacted Files	Coverage Δ
storage.go	51.38% <57.90%> (-0.85%)	⬇️
disk_wal.go	49.66% <70.00%> (-1.38%)	⬇️
wal.go	66.67% <0.00%> (+16.67%)	⬆️

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