Handle mon update completion actions even with update(s) is blocked

[TheBlueMatt]

If we complete a ChannelMonitorUpdate persistence but there are blocked ChannelMonitorUpdates in the channel, we'll skip all the post-monitor-update logic entirely. While its correct that we can't resume the channel (as it expected the monitor updates it generated to complete, even if they ended up blocked), the post-update actions are a channelmanager.rs concept - they cannot be tied to blocked updates because channelmanager.rs doesn't even see blocked updates.

This can lead to a channel getting stuck waiting on itself. In a production environment, an LDK user saw a case where:
(a) an MPP payment was received over several channels, let's call
them A + B.
(b) channel B got into AwaitingRAA due to unrelated operations,
(c) the MPP payment was claimed, with async monitor updating,
(d) the revoke_and_ack we were waiting on was delivered, but the
resulting ChannelMonitorUpdate was blocked due to the
pending claim having inserted an RAA-blocking action,
(e) the preimage ChannelMonitorUpdate generated for channel B
completed persistence, which did nothing due to the blocked
ChannelMonitorUpdate.
(f) the Event::PaymentClaimed event was handled but it, too,
failed to unblock the channel.

Instead, here, we simply process post-update actions when an update completes, even if there are pending blocked updates. We do not fully unblock the channel, of course.

[ldk-reviews-bot]

I've assigned @wpaulino 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 94.82759% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.33%. Comparing base (6d9c676) to head (eb5492f).

Files with missing lines	Patch %	Lines
lightning/src/ln/functional_test_utils.rs	84.61%	1 Missing and 5 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4236   +/-   ##
=======================================
  Coverage   89.32%   89.33%           
=======================================
  Files         180      180           
  Lines      138641   138714   +73     
  Branches   138641   138714   +73     
=======================================
+ Hits       123844   123922   +78     
+ Misses      12174    12167    -7     
- Partials     2623     2625    +2     

Flag	Coverage Δ
fuzzing	35.96% <2.32%> (-0.01%)	⬇️
tests	88.70% <94.82%> (+<0.01%)	⬆️

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