channeldb: explicitly store the FinalCltvDelta within the ContractTerm struct

In this commit, we move to explicitly storing a bit more information
within the invoice. Currently this information is already stored in the
payment request, but by storing it at this level, callers that may not
be in the state to fully decode a payment request can obtain this data.

We avoid a database migration by appending this data to the end of an
invoice. When decoding, we'll try to read out this extra information,
and simply return what we have if it isn't found.

channeldb/invoice_test.go

@@ -3,6 +3,7 @@ package channeldb
 import (
 	"crypto/rand"
 	"crypto/sha256"
+	prand "math/rand"
 	"reflect"
 	"testing"
 	"time"
@@ -24,6 +25,7 @@ func randInvoice(value lnwire.MilliSatoshi) (*Invoice, error) {
 		Terms: ContractTerm{
 			PaymentPreimage: pre,
 			Value:           value,
+			FinalCltvDelta:  uint16(prand.Int31()),
 		},
 	}
 	i.Memo = []byte("memo")
@@ -66,6 +68,7 @@ func TestInvoiceWorkflow(t *testing.T) {
 	fakeInvoice.PaymentRequest = []byte("")
 	copy(fakeInvoice.Terms.PaymentPreimage[:], rev[:])
 	fakeInvoice.Terms.Value = lnwire.NewMSatFromSatoshis(10000)
+	fakeInvoice.Terms.FinalCltvDelta = uint16(prand.Int31())
 
 	// Add the invoice to the database, this should succeed as there aren't
 	// any existing invoices within the database with the same payment