fix: make partition loading more efficient

[xushiyan]

Description

Fix the partition loading logic such that it

• does not skip relevant partitions
• makes file group loading parallelized

#47

How are the changes test-covered

• N/A
• Automated tests (unit and/or integration tests)
• Manual tests
  • Details are described below

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.10%. Comparing base (65d8ed4) to head (5713556).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
- Coverage   89.32%   89.10%   -0.23%     
==========================================
  Files          15       15              
  Lines         834      835       +1     
==========================================
- Hits          745      744       -1     
- Misses         89       91       +2     

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