生产连接的线程切换时，加个双检锁，确保线程状态准确

fine-druid/src/com/fr/third/alibaba/druid/pool/DruidDataSource.java

@@ -619,6 +619,8 @@ public class DruidDataSource extends DruidAbstractDataSource implements DruidDat
     }
 
     private void checkThread() throws SQLException {
+        if (!createConnectionThread.isStarted() && !destroyConnectionThread.isStarted()) {
+            synchronized (this) {//线程安全问题，加个双检锁
                 if (!createConnectionThread.isStarted() && !destroyConnectionThread.isStarted()) {
                     createConnectionThread.setStarted(true);
                     createConnectionThread.start();
@@ -631,6 +633,8 @@ public class DruidDataSource extends DruidAbstractDataSource implements DruidDat
                     }
                 }
             }
+        }
+    }
 
     public void init() throws SQLException {
         if (inited) {