Re-introduce send_payment_with_route with the new API

[TheBlueMatt]

We removed send_payment_with_route cause the API was inscruitable and hard to work with without double-payment bugs, but that doesn't mean the concept of sending with a route isn't useful. We currently let users do that by manually matching the payment_id in their router, but that's pretty annoying to deal with, and we can do it internally. We should just do it with the new API that's practical to use rather than the old APIError-based one.