channeldb+invoices: add ScanInvoices and integrate with InvoiceRegistry

This commit adds channeldb.ScanInvoices to scan through all invoices in
the database. The new call will also replace the already existing
channeldb.FetchAllInvoicesWithPaymentHash call in preparation to collect
invoices we'd like to delete and watch for expiry in one scan in later
commits.

invoices/invoice_expiry_watcher_test.go

@@ -158,24 +158,14 @@ func TestInvoiceExpiryWhenAddingMultipleInvoices(t *testing.T) {
 	t.Parallel()
 
 	test := newInvoiceExpiryWatcherTest(t, testTime, 5, 5)
-	var invoices []channeldb.InvoiceWithPaymentHash
+	invoices := make(map[lntypes.Hash]*channeldb.Invoice)
 
 	for hash, invoice := range test.testData.expiredInvoices {
-		invoices = append(invoices,
-			channeldb.InvoiceWithPaymentHash{
-				Invoice:     *invoice,
-				PaymentHash: hash,
-			},
-		)
+		invoices[hash] = invoice
 	}
 
 	for hash, invoice := range test.testData.pendingInvoices {
-		invoices = append(invoices,
-			channeldb.InvoiceWithPaymentHash{
-				Invoice:     *invoice,
-				PaymentHash: hash,
-			},
-		)
+		invoices[hash] = invoice
 	}
 
 	test.watcher.AddInvoices(invoices)