Dual-Funded Lightning Channel Construction Protocol

[Christewart]

I wanted to get this on the issue tracker as I feel at minimum there is a lot we can use from this, maybe at maximum we can copy it whole sale.

https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-January/002466.html

This would replaces parts of the Protocol.md spec.

[nkohen]

New updates pushed to niftynei/lightning-rfc#1