It seems like the amount given to the getBandwidth is 0 in some cases
(keysend) and non-zero in other cases (invoice payment).
If we use custom channels, then that amount will likely not be the
actual HTLC amount we're going to put on chain. So we have to use a zero
amount for the MayAddOutgoingHtlc check, otherwise we might seem to go
below the local channel reserve even though we'll end up sending a very
small amount only.
And since the check here is just to find out we're not already at the
maximum HTLC capacity for this channel, we can safely use the amount of
0 for the check, which is handled properly in the link logic.
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.
