Improve corpus cache behavior

[thomaskrause]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 67.01%. Comparing base (20c2e44) to head (923fae8).

Files	Patch %	Lines
core/src/graph/mod.rs	95.65%	3 Missing ⚠️
graphannis/src/annis/db/corpusstorage.rs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #293      +/-   ##
==========================================
- Coverage   67.06%   67.01%   -0.05%     
==========================================
  Files          76       76              
  Lines       19378    19449      +71     
  Branches    19378    19449      +71     
==========================================
+ Hits        12995    13034      +39     
- Misses       4975     5012      +37     
+ Partials     1408     1403       -5     

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