Hold times for successful payments

[joostjager]

Apply the hold time reporting mechanism as implemented in #2256 to the success case.

[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

Attention: Patch coverage is 98.50746% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.58%. Comparing base (22908ab) to head (69a99b4).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	93.10%	1 Missing and 1 partial ⚠️
lightning/src/ln/onion_utils.rs	99.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3801      +/-   ##
==========================================
+ Coverage   89.81%   90.58%   +0.76%     
==========================================
  Files         159      159              
  Lines      128672   134584    +5912     
  Branches   128672   134584    +5912     
==========================================
+ Hits       115571   121912    +6341     
+ Misses      10428    10067     -361     
+ Partials     2673     2605      -68     

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