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

@@ -620,14 +620,18 @@ public class DruidDataSource extends DruidAbstractDataSource implements DruidDat
 
     private void checkThread() throws SQLException {
         if (!createConnectionThread.isStarted() && !destroyConnectionThread.isStarted()) {
-            createConnectionThread.setStarted(true);
-            createConnectionThread.start();
-            destroyConnectionThread.setStarted(true);
-            destroyConnectionThread.start();
-            try {
-                initedLatch.await();
-            } catch (InterruptedException e) {
-                throw new SQLException(e.getMessage(), e);
+            synchronized (this) {//线程安全问题，加个双检锁
+                if (!createConnectionThread.isStarted() && !destroyConnectionThread.isStarted()) {
+                    createConnectionThread.setStarted(true);
+                    createConnectionThread.start();
+                    destroyConnectionThread.setStarted(true);
+                    destroyConnectionThread.start();
+                    try {
+                        initedLatch.await();
+                    } catch (InterruptedException e) {
+                        throw new SQLException(e.getMessage(), e);
+                    }
+                }
             }
         }
     }