LSPS2 Service doesn't enforce fees on later forwards

[TheBlueMatt]

After fixing #499, LSPS2 doesn't enforce fees on forwards after the first. In practice this means LDK needs full interception (maybe just for channels marked for it?), I think, so its time to add that, but in theory we could leave #499 un-fixed and then force clients to only relay through the interception SCID, but then it seems like we always open a new channel even if there's enough liquidity available? (presumably we should never open a new channel with a client if there's a channel with enough liquidity?)

[tnull]

Hmm, well, I think we could just allow to specify the 'regular' forwarding fees separately, no? In this initial alpha version of the service we just set them to 0 to make the calculations a bit easier, and to not charge them on top of the channel opening fees for the initial payment.

[tnull]

but then it seems like we always open a new channel even if there's enough liquidity available? (presumably we should never open a new channel with a client if there's a channel with enough liquidity?)

Yeah, there is two answers to this, really:

a) in the LDK Node service-side we need do proper tracking of the to-be-opened channels (retry them after reconnection, etc, see #473), but possibly also not to reopen another channel if the same intercept SCID is reused (though that is exactly the kind of things I'd love feedback from LSPs on: do they want a 1:1 mapping here or not?)

b) Eventually, all LSP operations should default to splicing-in rather than opening redudant channels, and then we could discuss whether to provide a specific API to still allow to request new additional channels, IMO.