Set holder_commitment_point to Available on upgrade

[TheBlueMatt]

When we upgrade from LDK 0.0.123 or prior, we need to intialize holder_commitment_point with commitment point(s). In 1f7f3a3 we changed the point(s) which we fetch from both the current and next per-commitment-point (setting the value to HolderCommitmentPoint::Available on upgrade) to only fetching the current per-commitment-point (setting the value to HolderCommitmentPoint::PendingNext on upgrade).

In commitment_signed handling, we expect the next per-commitment-point to be available (allowing us to advance() the holder_commitment_point), as it was included in the revoke_and_ack we most recently sent to our peer, so must've been available at that time.

Sadly, these two interact negatively with each other - on upgrade, assuming the channel is at a steady state and there are no pending updates, we'll not make the next per-commitment-point available but if we receive a commitment_signed from our peer we'll assume it is. As a result, in debug mode, we'll hit an assertion failure, and in production mode we'll force-close the channel.

Instead, here, we fix the upgrade logic to always upgrade directly to HolderCommitmentPoint::Available, making the next per-commitment-point available immediately.

We also attempt to resolve the next per-commitment-point in get_channel_reestablish, allowing any channels which were upgraded to LDK 0.0.124 and are in this broken state to avoid the force-closure, as long as they don't receive a commitment_signed in the interim.

This was included in the 0.0.125 release.

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.58%. Comparing base (46d8a0d) to head (1486b1b).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3365      +/-   ##
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- Coverage   89.61%   89.58%   -0.03%     
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  Files         127      127              
  Lines      103533   103544      +11     
  Branches   103533   103544      +11     
==========================================
- Hits        92778    92758      -20     
- Misses       8056     8082      +26     
- Partials     2699     2704       +5     

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