add Round-Robin Strategy to balancer (#2844)

* add Round-Robin Strategy

* clean up

infra/conf/router.go

@@ -43,6 +43,8 @@ func (r *BalancingRule) Build() (*router.BalancingRule, error) {
 		strategy = strategyRandom
 	case strategyLeastPing:
 		strategy = "leastPing"
+	case strategyRoundRobin:
+		strategy = "roundRobin"
 	default:
 		return nil, newError("unknown balancing strategy: " + r.Strategy.Type)
 	}

infra/conf/router_strategy.go

@@ -1,6 +1,7 @@
 package conf
 
 const (
-	strategyRandom    string = "random"
-	strategyLeastPing string = "leastping"
+	strategyRandom     string = "random"
+	strategyLeastPing  string = "leastping"
+	strategyRoundRobin string = "roundrobin"
 )