Support Splicing in LSPS2

[TheBlueMatt]

We shouldn't open a new channel with a client if we already have one. This may be a simple doc + ldk-node update, or there could be more, not sure.

[tnull]

This may be a simple doc + ldk-node update, or there could be more, not sure.

It is, especially since anything beyond just using splicing when you'd usually use a new channel wouldn't work out, cf the discussion over at lightning/blips#65. I'm going ahead and move this issue over to LDK Node.