multi: validate payment params at RPC layer

With this patch, we'll fail out earlier in the cycle in case of
some wonky parameters, and not leave zombie payments in the router
which currently are not cleaned up.

routing/payment_session_test.go

@@ -6,8 +6,70 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
+	"github.com/stretchr/testify/require"
 )
 
+func TestValidateCLTVLimit(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name           string
+		cltvLimit      uint32
+		finalCltvDelta uint16
+		includePadding bool
+		expectError    bool
+	}{
+		{
+			name:           "bad limit with padding",
+			cltvLimit:      uint32(103),
+			finalCltvDelta: uint16(100),
+			includePadding: true,
+			expectError:    true,
+		},
+		{
+			name:           "good limit with padding",
+			cltvLimit:      uint32(104),
+			finalCltvDelta: uint16(100),
+			includePadding: true,
+			expectError:    false,
+		},
+		{
+			name:           "bad limit no padding",
+			cltvLimit:      uint32(100),
+			finalCltvDelta: uint16(100),
+			includePadding: false,
+			expectError:    true,
+		},
+		{
+			name:           "good limit no padding",
+			cltvLimit:      uint32(101),
+			finalCltvDelta: uint16(100),
+			includePadding: false,
+			expectError:    false,
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+
+		success := t.Run(testCase.name, func(t *testing.T) {
+			err := ValidateCLTVLimit(
+				testCase.cltvLimit, testCase.finalCltvDelta,
+				testCase.includePadding,
+			)
+
+			if testCase.expectError {
+				require.NotEmpty(t, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+		if !success {
+			break
+		}
+	}
+}
+
 func TestRequestRoute(t *testing.T) {
 	const (
 		height = 10