Once ChannelManager supports payment retries, it will make more sense for its
current send_payment method to be named send_payment_with_route because
retrying should be the default. Here we get a head start on this by making the
rename in outbound_payment, but not changing the public interface yet.
This allows us to move a lot of outbound payment logic out of ChannelManager
and into the new outbound_payment module, and helps avoid growing
ChannelManager when we add retry logic to it in upcoming work.
We want to move all outbound payment-related things to this new module, to help
break up ChannelManager so future payment retries work doesn't increase the
size of ChannelManager.
