This commit was added to the 0-19-staging branch recently and therefore
didn't make it into a previous part yet. So it's unrelated to the
changes in this part but is required for the whole custom channel saga.
In preparation for structs outside of the `routing` package implementing
this interface, export `routingGraph` and rename it to `Graph` so as to
avoid stuttering.
Pass htlc amount down to the channel so that we don't need to rely
on minHtlc (and pad it when the channel sets a 0 min htlc). Update
test to just check some sane values since we're no longer relying
on minHtlc amount at all.
