[0.1] 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.

Backport of #4236 (that needs to go first so I can update the commit hash), plus a backport of #4172.

[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 93.12977% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.48%. Comparing base (4c1aa13) to head (21037ce).
⚠️ Report is 5 commits behind head on 0.1.

Files with missing lines	Patch %	Lines
lightning/src/ln/functional_test_utils.rs	86.04%	1 Missing and 5 partials ⚠️
lightning/src/ln/chanmon_update_fail_tests.rs	97.50%	2 Missing ⚠️
lightning/src/util/ser.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## 0.1 #4235 +/- ##
=======================================
Coverage 87.48% 87.48% =======================================
Files 149 149 Lines 101834 101924 +90 Branches 101834 101924 +90 =======================================
+ Hits 89092 89172 +80 - Misses 10479 10486 +7 - Partials 2263 2266 +3 

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