Avoid keeping lock while writing to disk cache

[glandium]

This splits the insertion operation into two separate operations, so that the lock can be released while performing I/O.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.72816% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 42.77%. Comparing base (0cc0c62) to head (12e16e7).
Report is 45 commits behind head on main.

Files	Patch %	Lines
src/lru_disk_cache/mod.rs	73.62%	4 Missing and 20 partials ⚠️
src/cache/disk.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2193       +/-   ##
===========================================
+ Coverage   30.91%   42.77%   +11.86%     
===========================================
  Files          53       53               
  Lines       20112    20412      +300     
  Branches     9755     9725       -30     
===========================================
+ Hits         6217     8731     +2514     
- Misses       7922     7982       +60     
+ Partials     5973     3699     -2274     

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