Support ChannelManager-less Phantom Invoice Generation

After #1199 lands we'll support generating "phantom" invoices which are invoices for payment that can be received from multiple nodes. Ideally you shouldn't need a live ChannelManager to generate such an invoice as long as you have the relevant ChannelDetails. This may need a refactoring of the payment secret and SCID generation logic to make it support "ChannelManager-less" operation.