refactor: combine actions counts in CheckpointVisitor

[sebastiantia]

resolves #876

We have no use for the counters in CheckpointVisitor: file_actions & non_file_actions. We combine the two downstream to retrieve the total actions_count when writing last_checkpoint.

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.11%. Comparing base (399baf5) to head (e35f4f1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/checkpoint/log_replay.rs	88.23%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #883      +/-   ##
==========================================
- Coverage   85.11%   85.11%   -0.01%     
==========================================
  Files          87       87              
  Lines       21874    21865       -9     
  Branches    21874    21865       -9     
==========================================
- Hits        18619    18610       -9     
  Misses       2298     2298              
  Partials      957      957              

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[zachschuermann]

LGTM, thanks!

[nicklan]

thanks!