Rustfmt parts of channelmanager.rs

[joostjager]

Prepare for #3957

[ldk-reviews-bot]

I've assigned @tankyleo 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 79.41176% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.93%. Comparing base (55baa15) to head (0d56fb7).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	79.41%	27 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #3960 +/- ##
==========================================
- Coverage 88.94% 88.93% -0.02% 
==========================================
Files 173 173 Lines 123794 123876 +82 Branches 123794 123876 +82 ==========================================
+ Hits 110114 110165 +51 - Misses 11227 11255 +28 - Partials 2453 2456 +3 

Flag	Coverage Δ
fuzzing	22.17% <0.00%> (-0.04%)	⬇️
tests	88.75% <79.41%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tankyleo]

Reproduced both commits on my machine.

[ldk-reviews-bot]

👋 The first review has been submitted!

Do you think this PR is ready for a second reviewer? If so, click here to assign a second reviewer.