routing: add outgoing channel restriction #2572
This PR adds the ability to SendPayment to restrict the payment to a specific outgoing channel.
One of the use cases of this is to be able to balance channels passively. Paying a higher fee for a payment, but at the same time increasing the receive capacity of a specific channel.
This is an alternative to making a dedicated payment to yourself to achieve the same goal, which may overall be more costly in terms of routing fees.