blindedpath: smarter dummy hop policy selection

This commit introduces more sophisticated code for selecting dummy hop
policy values for dummy hops in blinded paths.

For the case where the path does contain real hops, the dummy hop policy
values are derived by taking the average of those hop polices. For the
case where there are no real hops (in other words, we are the
introduction node), we use the default policy values used for normal
ChannelUpdates but then for the MaxHTLC value, we take the average of
all our open channel capacities.

routing/blindedpath/blinded_path_test.go

@@ -802,7 +802,7 @@ func TestBuildBlindedPathWithDummyHops(t *testing.T) {
 		// hops to be added to the real route.
 		MinNumHops: 4,
 
-		DummyHopPolicy: &BlindedHopPolicy{
+		DefaultDummyHopPolicy: &BlindedHopPolicy{
 			CLTVExpiryDelta: 50,
 			FeeRate:         100,
 			BaseFee:         100,
@@ -817,8 +817,8 @@ func TestBuildBlindedPathWithDummyHops(t *testing.T) {
 
 	// Check that all the accumulated policy values are correct.
 	require.EqualValues(t, 403, path.FeeBaseMsat)
-	require.EqualValues(t, 1203, path.FeeRate)
-	require.EqualValues(t, 400, path.CltvExpiryDelta)
+	require.EqualValues(t, 2003, path.FeeRate)
+	require.EqualValues(t, 588, path.CltvExpiryDelta)
 	require.EqualValues(t, 1000, path.HTLCMinMsat)
 	require.EqualValues(t, lnwire.MaxMilliSatoshi, path.HTLCMaxMsat)
 
@@ -861,7 +861,7 @@ func TestBuildBlindedPathWithDummyHops(t *testing.T) {
 	}, data.RelayInfo.UnwrapOrFail(t).Val)
 
 	require.Equal(t, record.PaymentConstraints{
-		MaxCltvExpiry:   1600,
+		MaxCltvExpiry:   1788,
 		HtlcMinimumMsat: 1000,
 	}, data.Constraints.UnwrapOrFail(t).Val)
 
@@ -883,7 +883,7 @@ func TestBuildBlindedPathWithDummyHops(t *testing.T) {
 	}, data.RelayInfo.UnwrapOrFail(t).Val)
 
 	require.Equal(t, record.PaymentConstraints{
-		MaxCltvExpiry:   1456,
+		MaxCltvExpiry:   1644,
 		HtlcMinimumMsat: 1000,
 	}, data.Constraints.UnwrapOrFail(t).Val)