0conf LN channels and v3 anchors

[morehouse]

@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.

Without complicating the current v3 proposal, we could achieve similar results by making 0-conf funding transactions v3 and adding a shared anchor to them. Then either party can CPFP in an emergency.