Hold in-flight monitor updates until background event processing

[wpaulino]

We previously assumed background events would eventually be processed prior to another ChannelManager write, so we would immediately remove all in-flight monitor updates that completed since the last ChannelManager serialization. This isn't always the case, so we now keep them all around until we're ready to handle them, i.e., when process_background_events is called.

This was discovered while fuzzing chanmon_consistency_target on the main branch with some changes that allow it to connect blocks. It was triggered by reloading the ChannelManager after a monitor update completion for an outgoing HTLC, calling ChannelManager::best_block_updated, and reloading the ChannelManager once again. A test is included that provides a minimal reproduction of this case.

[ldk-reviews-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.01%. Comparing base (817ab5e) to head (2a15c45).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	88.23%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4377      +/-   ##
==========================================
- Coverage   86.01%   86.01%   -0.01%     
==========================================
  Files         156      156              
  Lines      102857   102869      +12     
  Branches   102857   102869      +12     
==========================================
+ Hits        88476    88483       +7     
- Misses      11871    11874       +3     
- Partials     2510     2512       +2     

Flag	Coverage Δ
tests	86.01% <88.23%> (-0.01%)	⬇️

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