RoundRobinLoadBalancer enhancement (vesoft-inc#357)

* send yield command as ping, for fail fast
* use ConcurrentHashMap instead of HashMap, to avoid volatile problem

client/src/main/java/com/vesoft/nebula/client/graph/net/RoundRobinLoadBalancer.java

@@ -5,9 +5,9 @@
 import com.vesoft.nebula.client.graph.exception.ClientServerIncompatibleException;
 import com.vesoft.nebula.client.graph.exception.IOErrorException;
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
@@ -20,7 +20,7 @@ public class RoundRobinLoadBalancer implements LoadBalancer {
     private static final int S_OK = 0;
     private static final int S_BAD = 1;
     private final List<HostAddress> addresses = new ArrayList<>();
-    private final Map<HostAddress, Integer> serversStatus = new HashMap<>();
+    private final Map<HostAddress, Integer> serversStatus = new ConcurrentHashMap<>();
     private final int timeout;
     private final AtomicInteger pos = new AtomicInteger(0);
     private final int delayTime = 60;  // unit seconds
@@ -80,8 +80,9 @@ public boolean ping(HostAddress addr) {
             } else {
                 connection.open(addr, this.timeout);
             }
+            boolean pong = connection.ping();
             connection.close();
-            return true;
+            return pong;
         } catch (IOErrorException | ClientServerIncompatibleException e) {
             return false;
         }