routing: correct fee calculation when converting from path to route
routing: correct fee calculation when converting from path to route

In this commit, we correct the fee calculation when converting from a
path to route. Previously we would apply the “no fee” case at the
_first_ hop, rather than the last hop. As a result, we needed to swap
the edges during path finding, otherwise, if the incoming and outgoing
edges had different fee rates, then we would create an invalid onion
payload.

In this commit we now properly switch fee calculation into three cases:
  * a single hop route, so there’s no fee
  * we’re at the first hop in a multi hop route, and we apply the fee
for the _next_ hop
  * we’re at an intermediate hop and the fee calculation proceeds as
normal

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd