invoices: adding InvoiceExpryWatcher to cancel expired invoices

This commit adds InvoiceExpryWatcher which is a separate class that
receives new invoices (and existing ones upon restart) from InvoiceRegistry
and actively watches their expiry. When an invoice is expired
InvoiceExpiryWatcher will call into InvoiceRegistry to cancel the
invoice and by that notify all subscribers about the state change.

zpay32/invoice.go

@@ -82,6 +82,10 @@ const (
 	// This is chosen to be the maximum number of bytes that can fit into a
 	// single QR code: https://en.wikipedia.org/wiki/QR_code#Storage
 	maxInvoiceLength = 7089
+
+	// DefaultInvoiceExpiry is the default expiry duration from the creation
+	// timestamp if expiry is set to zero.
+	DefaultInvoiceExpiry = time.Hour
 )
 
 var (