multi: introduce new traffic shaper method.

We introduce a new specific fail resolution error when the
external HTLC interceptor denies the incoming HTLC. Moreover
we introduce a new traffic shaper method which moves the
implementation of asset HTLC to the external layers.
Moreover itests are adopted to reflect this new change.

itest/list_on_test.go

@@ -406,14 +406,6 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "forward interceptor",
 		TestFunc: testForwardInterceptorBasic,
 	},
-	{
-		Name:     "forward interceptor modified htlc",
-		TestFunc: testForwardInterceptorModifiedHtlc,
-	},
-	{
-		Name:     "forward interceptor wire records",
-		TestFunc: testForwardInterceptorWireRecords,
-	},
 	{
 		Name:     "forward interceptor restart",
 		TestFunc: testForwardInterceptorRestart,