Fix timing of cache reads. Fixes #1817

[luser]

I'm not sure when this got broken, but the cache read timing isn't being calculated properly. This change fixes it. I added a test by extending the MockStorage struct (which is only used in one other test) to include an optional delay on get, and just asserting that the cache read duration is >= that delay. Fixes #1817

[codecov-commenter]

Codecov Report

Patch coverage: 30.00% and project coverage change: +0.01 🎉

Comparison is base (62c8f0a) 29.48% compared to head (0613eb1) 29.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1818      +/-   ##
==========================================
+ Coverage   29.48%   29.49%   +0.01%     
==========================================
  Files          49       49              
  Lines       17780    17843      +63     
  Branches     8592     8637      +45     
==========================================
+ Hits         5242     5263      +21     
- Misses       7353     7358       +5     
- Partials     5185     5222      +37     

Impacted Files	Coverage Δ
src/test/mock_storage.rs	25.80% <28.57%> (-1.12%)	⬇️
src/compiler/compiler.rs	35.63% <30.15%> (-0.12%)	⬇️

... and 14 files with indirect coverage changes

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[sylvestre]

welcome back @luser ;)