Account for dust in closing channel fee

[benthecarman]

Before we were ignoring dust when creating the closing channel fee. Now if an output would be dust, we will recalculate the fee with the missing output. This should fix some force closes that happen with lnd nodes.

Draft for now because no tests, couldn't find if this function was tested already

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (f068df0) 90.37% compared to head (a2741a0) 90.38%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2344      +/-   ##
==========================================
+ Coverage   90.37%   90.38%   +0.01%     
==========================================
  Files         104      104              
  Lines       53779    53787       +8     
  Branches    53779    53787       +8     
==========================================
+ Hits        48601    48618      +17     
+ Misses       5178     5169       -9     

Impacted Files	Coverage Δ
lightning/src/ln/channel.rs	89.71% <100.00%> (+0.02%)	⬆️

... and 4 files with indirect coverage changes

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[TheBlueMatt]

While its probably true that we need to update the code here as well, I don't think we handle the "remove my output cause its small" case in closing_signed either - we build a closing_tx based on the removal of the counterparty's output, but we don't adjust the fee range accordingly - we should be willing to pay a much higher fee if they removed their output, cause its not our money.

[TheBlueMatt]

Let's tackle this as a part of #2433 (which removes all shutdown FCs). I'm kinda hoping we can do that, get LND to do it too, and then never have to think about the old fee negotiation code again (and hopefully rip it out entirely in a year or two, just force-close always if the peer doesn't support the sensible approach). If you want to pick this back up just ping and I'm happy to reopen it, but (a) I'm not super convinced this is a common force-closure reason - more commonly both sides want their balance but one side just sends a nonsense fee, usually because of a Bitcoin Core crash or restart and (b) I hate this code and all the close negotiation code and am excited to never look at it again so I'm gonna tentatively close this.