Routes contain only routing information and the payment they're linked
to can be obtained through the payment_hash. We remove the dependency of
route building routines from the payment itself. In order to make
plain payment information available we define a payment_info structure.
Add a new implementation of the payment state machine.
This is based in the `pay` plugin concept of payment modifiers,
but here we take it to the next level.
The payment goes through a virtual machine that includes calling
functions and evaluating conditions.
