Refactor issue indexer (#5363)

modules/indexer/issues/queue.go

@@ -0,0 +1,11 @@
+// Copyright 2018 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package issues
+
+// Queue defines an interface to save an issue indexer queue
+type Queue interface {
+	Run() error
+	Push(*IndexerData)
+}