Break up and function-ize handle_new_monitor_update

[valentinewallace]

Prefactor to some of @joostjager's experimental work including serialized channels in ChannelMonitorUpdates as part of getting rid of ChannelManager persistence.

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[codecov]

Codecov Report

❌ Patch coverage is 60.60606% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.60%. Comparing base (54ed941) to head (c7e5c70).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	60.60%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4138      +/-   ##
==========================================
- Coverage   88.62%   88.60%   -0.02%     
==========================================
  Files         180      180              
  Lines      134895   135247     +352     
  Branches   134895   135247     +352     
==========================================
+ Hits       119546   119839     +293     
- Misses      12587    12643      +56     
- Partials     2762     2765       +3     

Flag	Coverage Δ
fuzzing	21.76% <39.39%> (+<0.01%)	⬆️
tests	88.44% <60.60%> (-0.02%)	⬇️

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