perf: use memmap to speed up file reading

[Boshen]

No description provided.

[graphite-app]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.92%. Comparing base (19c4868) to head (b76115b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #696      +/-   ##
==========================================
+ Coverage   94.89%   94.92%   +0.02%     
==========================================
  Files          12       12              
  Lines        2882     2897      +15     
==========================================
+ Hits         2735     2750      +15     
  Misses        147      147              

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[codspeed-hq]

CodSpeed Performance Report

Merging #696 will not alter performance

_{Comparing mmap2 (b76115b) with main (19c4868)¹}

Summary

✅ 7 untouched
⏩ 2 skipped²

Footnotes

1. No successful run was found on main (49cc7d0) during the generation of this report, so 19c4868 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩