Fake SCID/Channel API

[TheBlueMatt]

We need a way to create fake SCIDs for downstream channels, and also ideally a way to create a fake last-hop in invoices which doesn't actually exist, accepting the payment instead. This may depend on #680.

[TheBlueMatt]

Moving to 0.0.101 - we need to have this for some users that want a cluster of nodes but where users can send payment to any of the cluster members. Should be easy, though.

[TheBlueMatt]

We shipped this a while ago.