Convert internal update handling fns to methods

[joostjager]

Small readability improvement

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 90.56604% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.38%. Comparing base (62c5849) to head (dfbf531).

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	90.56%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4291   +/-   ##
=======================================
  Coverage   89.38%   89.38%           
=======================================
  Files         180      180           
  Lines      139834   139836    +2     
  Branches   139834   139836    +2     
=======================================
+ Hits       124985   124989    +4     
+ Misses      12262    12259    -3     
- Partials     2587     2588    +1     

Flag	Coverage Δ
fuzzing	35.22% <75.47%> (+<0.01%)	⬆️
tests	88.71% <86.79%> (-0.01%)	⬇️

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