reduce source pressure when use scan (#823)

* update else if to switch

* update else if to switch

* 添加动态识别库号

* 性能优化

* 添加scan时候的一次扫描次数控制配置，防止源端cpu高

* rename batch to count

* edit ...string to ...interface

* performance optimization

* fix r.db length is 0

internal/reader/scan_standalone_reader.go

@@ -101,11 +101,15 @@ func (r *scanStandaloneReader) StartRead(ctx context.Context) chan *entry.Entry
 
 func (r *scanStandaloneReader) subscript() {
 	c := client.NewRedisClient(r.ctx, r.opts.Address, r.opts.Username, r.opts.Password, r.opts.Tls)
-	c.Send("psubscribe", "__keyevent@*__:*")
-	// filter dbs
-	dbIDmap := make(map[int]struct{})
-	for _, db := range r.dbs {
-		dbIDmap[db] = struct{}{}
+	if len(r.dbs) == 0 {
+		c.Send("psubscribe", "__keyevent@*__:*")
+	} else {
+		strs := make([]string, len(r.dbs))
+		for i, v := range r.dbs {
+			strs[i] = strconv.Itoa(v)
+		}
+		s := fmt.Sprintf("__keyevent@[%v]__:*", strings.Join(strs, ","))
+		c.Send("psubscribe", s)
 	}
 	_, err := c.Receive()
 	if err != nil {
@@ -130,20 +134,16 @@ func (r *scanStandaloneReader) subscript() {
 			if err != nil {
 				log.Panicf(err.Error())
 			}
-			// if the db is not in the dbs, ignore it
-			if _, ok := dbIDmap[dbIdInt]; ok {
-				// handle del action
-				eventSlice := strings.Split(respSlice[2].(string), ":")
-				if eventSlice[1] == "del" {
-					e := entry.NewEntry()
-					e.DbId = dbIdInt
-					e.Argv = []string{"DEL", key}
-					r.ch <- e
-					continue
-				}
-
-				r.needDumpQueue.Put(dbKey{db: dbIdInt, key: key})
+			// handle del action
+			eventSlice := strings.Split(respSlice[2].(string), ":")
+			if eventSlice[1] == "del" {
+				e := entry.NewEntry()
+				e.DbId = dbIdInt
+				e.Argv = []string{"DEL", key}
+				r.ch <- e
+				continue
 			}
+			r.needDumpQueue.Put(dbKey{db: dbIdInt, key: key})
 		}
 	}
 }
@@ -200,7 +200,7 @@ func (r *scanStandaloneReader) dump() {
 		r.dumpClient.Do("READONLY")
 		log.Infof("running dump() in read-only mode")
 	}
-	
+
 	for item := range r.needDumpQueue.Ch {
 		r.stat.NeedUpdateCount = int64(r.needDumpQueue.Len())
 		dbId := item.(dbKey).db