multi: thread bandwidth check amount down to MayAddOutgoingHtlc

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.

routing/integrated_routing_context_test.go

@@ -22,8 +22,8 @@ type mockBandwidthHints struct {
 	hints map[uint64]lnwire.MilliSatoshi
 }
 
-func (m *mockBandwidthHints) availableChanBandwidth(channelID uint64) (
-	lnwire.MilliSatoshi, bool) {
+func (m *mockBandwidthHints) availableChanBandwidth(channelID uint64,
+	_ lnwire.MilliSatoshi) (lnwire.MilliSatoshi, bool) {
 
 	if m.hints == nil {
 		return 0, false