htlcswitch: obtain the min final CLTV delta directly from the InvoiceDatabase

In this commit, we modify the existing logic that would attempt to read
the min CLTV information from the invoice directly. With this route, we
avoid any sort of DB index modifications, as this information is already
stored within the payment request, which is already available to the
outside callers. By modifying the InvoiceDatabase interface, we avoid
having to make the switch aware of what the "primary" chain is.

htlcswitch/mock.go

@@ -149,7 +149,7 @@ func initSwitchWithDB(startingHeight uint32, db *channeldb.DB) (*Switch, error)
 }
 
 func newMockServer(t testing.TB, name string, startingHeight uint32,
-	db *channeldb.DB) (*mockServer, error) {
+	db *channeldb.DB, defaultDelta uint32) (*mockServer, error) {
 
 	var id [33]byte
 	h := sha256.Sum256([]byte(name))
@@ -166,7 +166,7 @@ func newMockServer(t testing.TB, name string, startingHeight uint32,
 		name:             name,
 		messages:         make(chan lnwire.Message, 3000),
 		quit:             make(chan struct{}),
-		registry:         newMockRegistry(),
+		registry:         newMockRegistry(defaultDelta),
 		htlcSwitch:       htlcSwitch,
 		interceptorFuncs: make([]messageInterceptor, 0),
 	}, nil
@@ -648,25 +648,29 @@ var _ ChannelLink = (*mockChannelLink)(nil)
 
 type mockInvoiceRegistry struct {
 	sync.Mutex
-	invoices map[chainhash.Hash]channeldb.Invoice
+
+	invoices   map[chainhash.Hash]channeldb.Invoice
+	finalDelta uint32
 }
 
-func newMockRegistry() *mockInvoiceRegistry {
+func newMockRegistry(minDelta uint32) *mockInvoiceRegistry {
 	return &mockInvoiceRegistry{
-		invoices: make(map[chainhash.Hash]channeldb.Invoice),
+		finalDelta: minDelta,
+		invoices:   make(map[chainhash.Hash]channeldb.Invoice),
 	}
 }
 
-func (i *mockInvoiceRegistry) LookupInvoice(rHash chainhash.Hash) (channeldb.Invoice, error) {
+func (i *mockInvoiceRegistry) LookupInvoice(rHash chainhash.Hash) (channeldb.Invoice, uint32, error) {
 	i.Lock()
 	defer i.Unlock()
 
 	invoice, ok := i.invoices[rHash]
 	if !ok {
-		return channeldb.Invoice{}, fmt.Errorf("can't find mock invoice: %x", rHash[:])
+		return channeldb.Invoice{}, 0, fmt.Errorf("can't find mock "+
+			"invoice: %x", rHash[:])
 	}
 
-	return invoice, nil
+	return invoice, i.finalDelta, nil
 }
 
 func (i *mockInvoiceRegistry) SettleInvoice(rhash chainhash.Hash) error {