Async Payjoin.

Would let a payer and receiver build a joint Payjoin payment without both needing to be online at the same time.

The first Payjoin version needed the receiver to run a secure web server, with both sides online at once. This version passes encrypted messages through a go-between server that cannot read them or link them to users' internet addresses. Payjoin mixes both people's coins in one payment, helping privacy and saving fees.