Add more tests and docs for issue indexer, add db indexer type for searching from database (#6144)

* add more tests and docs for issue indexer, add db indexer type for searching from database

* fix typo

* fix typo

* fix lint

* improve docs

modules/indexer/issues/queue.go

@@ -7,5 +7,19 @@ package issues
 // Queue defines an interface to save an issue indexer queue
 type Queue interface {
 	Run() error
-	Push(*IndexerData)
+	Push(*IndexerData) error
+}
+
+// DummyQueue represents an empty queue
+type DummyQueue struct {
+}
+
+// Run starts to run the queue
+func (b *DummyQueue) Run() error {
+	return nil
+}
+
+// Push pushes data to indexer
+func (b *DummyQueue) Push(*IndexerData) error {
+	return nil
 }