Refactor handling of late-arriving COMMITs

[masih]

Move the handling of late-arriving COMMITs into switch statement for consistent code style and better readability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.87%. Comparing base (e3a71f8) to head (b2c22ec).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #609      +/-   ##
==========================================
- Coverage   79.50%   78.87%   -0.63%     
==========================================
  Files          52       52              
  Lines        4630     4630              
==========================================
- Hits         3681     3652      -29     
- Misses        591      613      +22     
- Partials      358      365       +7     

Files with missing lines	Coverage Δ
gpbft/gpbft.go	87.24% <100.00%> (ø)

... and 5 files with indirect coverage changes