Fix a bug when client reconnect (#2135)

* Add reconnection lock to control only one thread can can reconnect method.
This will avoid problem in this case:
Thread A reconnecting, and success, then send msg.
Thread B reconnecting but invoke disconnect. Then the Thread A will send msg fail because of Thread B's disconnecting call.

* fix sth

dubbo-remoting/dubbo-remoting-api/src/main/java/org/apache/dubbo/remoting/transport/AbstractClient.java

@@ -321,8 +321,17 @@ public void disconnect() {
 
     @Override
     public void reconnect() throws RemotingException {
-        disconnect();
-        connect();
+        if (!isConnected()) {
+            connectLock.lock();
+            try {
+                if (!isConnected()) {
+                    disconnect();
+                    connect();
+                }
+            } finally {
+                connectLock.unlock();
+            }
+        }
     }
 
     @Override