Add helper for getting claimable balance

[benthecarman]

It is annoying to have to match across all the enums of Balance to just pull out the claimable_amount_satoshis value. This helper makes it easier if you just want to amount.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.03 🎉

Comparison is base (fb140b5) 90.34% compared to head (06c6750) 90.37%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2333      +/-   ##
==========================================
+ Coverage   90.34%   90.37%   +0.03%     
==========================================
  Files         104      104              
  Lines       53392    53782     +390     
  Branches    53392    53782     +390     
==========================================
+ Hits        48237    48607     +370     
- Misses       5155     5175      +20     

Impacted Files	Coverage Δ
lightning/src/chain/channelmonitor.rs	94.73% <0.00%> (-0.16%)	⬇️

... and 23 files with indirect coverage changes

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[TheBlueMatt]

Hmmmmm, I want to better understand the use-case here - are you intending to use the Balance set to tell the user what their deposit balance is? If so, we probably want to only include one of MaybePreimageClaimableHTLC and MaybeTimeoutClaimableHTLC - if we've forwarded a payment, we're only going to get one of those, but never both (choosing which would mean deciding if you want to count outbound pending payments or not). Further, in that case we should probably include the commitment tx fee on Balance::ClaimableOnChannelClose as its a bit confusing to exclude that.

Lack of a global "what is my total balance, man" method is something we've needed for a while, but I'm not sure ChannelMonitors are the right place for it, rather ChannelManager has a bit more context, specifically in-flight payments that are pending a retry but haven't been retried yet.

[benthecarman]

The use case is basically to get rid of this giant match statement here: https://github.com/MutinyWallet/mutiny-node/blob/2c9436bd5eca5916d3c3812034314d97abcda124/mutiny-core/src/nodemanager.rs#L1025

We want to see the amount we are going to get back from force closes

[TheBlueMatt]

Mmm, then we probably want to 0 out the MaybePreimageClaimableHTLC branch? We'll want to carefully document what this is useful for and what assumptions it makes. If we dont know a preimage probably the counterparty will just claim it with the timeout.

[benthecarman]

Yeah maybe this is better as a function to the channel manager as something like get_limbo_balance that gets all the balance waiting in force closes.

Edit: doesn't seem possible because we don't have the chain monitor available :(

[benthecarman]

Excluded MaybePreimageClaimableHTLC and MaybeTimeoutClaimableHTLC and updated the documentation.