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/bandwidth_test.go

@@ -119,7 +119,7 @@ func TestBandwidthManager(t *testing.T) {
 			require.NoError(t, err)
 
 			bandwidth, found := m.availableChanBandwidth(
-				testCase.channelID,
+				testCase.channelID, 10,
 			)
 			require.Equal(t, testCase.expectedBandwidth, bandwidth)
 			require.Equal(t, testCase.expectFound, found)