refactor: `LFUCache`

[alxkm]

trafficstars

Refactoring LFUCache. Javadocs improvements. Readability improvements.

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 46.04%. Comparing base (d7b60be) to head (ea37d88).

Files	Patch %	Lines
.../thealgorithms/datastructures/caches/LFUCache.java	75.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5369      +/-   ##
============================================
- Coverage     46.04%   46.04%   -0.01%     
  Complexity     2865     2865              
============================================
  Files           525      525              
  Lines         15412    15412              
  Branches       2914     2916       +2     
============================================
- Hits           7097     7096       -1     
  Misses         8020     8020              
- Partials        295      296       +1     

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