Refactor issue indexer (#5363)

vendor/github.com/lunny/levelqueue/error.go

@@ -0,0 +1,12 @@
+// Copyright 2019 Lunny Xiao. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package levelqueue
+
+import "errors"
+
+var (
+	// ErrNotFound means no element in queue
+	ErrNotFound = errors.New("no key found")
+)