0conf LN channels and v3 anchors

[t-bast]

@MattCorallo

I’m not sure what the correct fix is for tweaking v3 to allow this, maybe the funding transaction could be restricted to no unconfirmed inputs and then we simply allow an A → B → {C, D, CPFP bump} topology.

I’m not sure that’s sufficient, because you usually want to allow having a chain of unconfirmed splices. The topologies that v3 would then need to support would probably be too complex to analyze before cluster mempool…

@MattCorallo

If the funding transaction confirms on its own (or via CPFP if B has a change output on it, though that’s certainly not a requirement we’d like to require) then we’re fine

My plan was to have a change output on the funding/splice transaction on B’s side to ensure that it could be CPFP-ed. This is indeed far from ideal, but that’s something we already need today if we want to protect against pinning, so v3 doesn’t make it worse?