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/mock_test.go

@@ -760,6 +760,6 @@ func (m *mockLink) EligibleToForward() bool {
 }
 
 // MayAddOutgoingHtlc returns the error configured in our mock.
-func (m *mockLink) MayAddOutgoingHtlc() error {
+func (m *mockLink) MayAddOutgoingHtlc(_ lnwire.MilliSatoshi) error {
 	return m.mayAddOutgoingErr
 }