Filter route hints for phantom invoices

Filter route hints for phantom invoices

Filter the route hints in `create_phantom_invoice` based on the
following criteria:

* Only one channel for every counterparty node per phantom
payment-receiving node in the invoice
* Always select the channel with the highest inbound capacity
* For each payment-receiving node, filter out channels with a lower
inbound capacity than the invoice amount, if any channel exists with
enough capacity to cover the invoice amount
* If any public channels exists for a payment-receiving node, push a
single RouteHintHop with the phantom route and let the sender find the
path to the payment-receiving node through the public channels.