Handle nodes with multiple channels between them better in scoring

[TheBlueMatt]

Currently we treat them each as distinct edges with different available liquidity, but this isn't how it works in practice - nodes will just forward over any channel they have. Maybe splicing will just fix this for us, but we might consider handling this somehow in the scorer by ignoring redundant channels and just treating it as one big channel.

[joostjager]

I can comment on how this works in lnd. In lnd, the level at which scoring is applied is a directed node pair. Indeed for the reason you mention, that nodes just forward over any channel they have.