Splicing out funds should update router balances

[pm47]

Otherwise we will run into unexpected InsufficientFunds errors when relaying payments.

[remyers]

I looked at this as part of PR #2936. It appears when the availableBalanceForSend amount changes after a splice, it will generate an AvailableBalanceChanged event that updates channel balances on the router.

Could this have already been fixed in a different PR? or am I missing something about the situation that causes this issue?

[t-bast]

It appears when the availableBalanceForSend amount changes after a splice,

I think this isn't sufficient: the code you're linking isn't triggered after a splice, it will only be triggered if we receive a new commit_sig after a splice has been confirmed (for example because an HTLC is added to the channel). But before that happens, we will still be using the pre-splice balances, which is an issue.

What we should probably do is emit an AvailableBalanceChanged event once we've exchanged splice_locked and are pruning the pre-splice commitments. I don't know if this should be part of #2936 or is independent.

[t-bast]

I'm pretty sure this is fixed by #2968 (probably even already fixed in one of the previous PRs).