Support Cluster IP

[gangtao]

For proton cluster, need to support cluster ip addresses

1. make sure cluster ip works

   conn := proton.OpenDB(&proton.Options{
       Addr: []string{"ip1:8463","ip2:8463,"ip3:8463"},
       Auth: proton.Auth{
           Username: "default",
           Password: "",
       },
   })
   var value int
   conn.QueryRow("SELECT 300").Scan(&value)
   fmt.Println(value)

2. client level load balancer

implement a round robin based query load balancing