Cache finances balance

[psikomonkie]

My MekHQ was running slow - taking over 10 seconds to process a new day for a thicc Battalion while off contract. IntelliJ's profiler suggested that Finances::getBalance was taking a lot of time, so it's now cached.

Before caching:

After caching:

[psikomonkie]

Hey @IllianiCBT this is that PR we briefly chatted about!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.36%. Comparing base (3f39021) to head (0087e25).
Report is 14 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6028      +/-   ##
============================================
+ Coverage     10.34%   10.36%   +0.01%     
- Complexity     6135     6150      +15     
============================================
  Files          1039     1040       +1     
  Lines        139412   139446      +34     
  Branches      20677    20684       +7     
============================================
+ Hits          14420    14450      +30     
+ Misses       123549   123542       -7     
- Partials       1443     1454      +11     

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