fix: Flush buffers before renaming cache file

[kuznetsss]

If clio shuts itself down due to exceeding graceful period when cache is saved and renamed but the buffers are not flushed, we may end up with a corrupted cache file. Clio will detect corruption and will not load corrupted cache file, but we could avoid it by explicitly flushing ofstream buffer.

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.02%. Comparing base (a72e5a1) to head (5a05813).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/data/impl/LedgerCacheFile.cpp	0.00%	1 Missing and 1 partial ⚠️
src/data/impl/OutputFile.cpp	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2927      +/-   ##
===========================================
- Coverage    82.04%   82.02%   -0.02%     
===========================================
  Files          391      391              
  Lines        14623    14630       +7     
  Branches      8001     8005       +4     
===========================================
+ Hits         11997    12000       +3     
- Misses        1447     1449       +2     
- Partials      1179     1181       +2     

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