invoices: refactor - add interface for expiry items

In preparation for having more than one expiry type, we
alias the queue.PrioirtyQueueItem interface for readability.

invoices/invoice_expiry_watcher_test.go

@@ -157,7 +157,7 @@ func TestInvoiceExpiryWhenAddingMultipleInvoices(t *testing.T) {
 	t.Parallel()
 
 	test := newInvoiceExpiryWatcherTest(t, testTime, 5, 5)
-	var invoices []*invoiceExpiryTs
+	var invoices []invoiceExpiry
 
 	for hash, invoice := range test.testData.expiredInvoices {
 		invoices = append(invoices, makeInvoiceExpiry(hash, invoice))