When doing the random walk through the channel, we now add the fees
(both the base and the proportional one) for that channel in addition to
the cltv delta.

Changelog-Added: Payment amount fuzzing is restored, but through shadow routing.

As Rusty pointed out to me, the gossip protocol restricts cltvs to u16
so at least we'll use this helper for them.

Had to make a special pylightning function to avoid rewriting all calls
to 'pay()' with 'rpc.call()' in the next commit..

So that we can assert payment values