Improve memory load for map_complete_blocks

[stephenworsley]

🚀 Pull Request

Addresses #3808.

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[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (8f0b62b) to head (f23a711).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/_lazy_data.py	63.63%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6730      +/-   ##
==========================================
- Coverage   90.29%   90.27%   -0.03%     
==========================================
  Files          91       91              
  Lines       24465    24493      +28     
  Branches     4568     4576       +8     
==========================================
+ Hits        22090    22110      +20     
- Misses       1607     1614       +7     
- Partials      768      769       +1     

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