This commit updates the traffic shaper's bandwidth query method to
accept the link bandwidth as a parameter. This allows an external
traffic shaper to optionally return the link bandwidth as the payment
bandwidth.
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.
