Made MongodbHandlerPool a singleton again

cdplib/db_handlers/MongodbHandler.py

@@ -24,8 +24,8 @@ from cdplib.log import Log
 from cdplib.db_migration.ParseJsonSchema import ParseJsonSchema
 
 
-#class MongodbHandlerPool(metaclass=SingletonThreadsafe):
-class MongodbHandlerPool():
+class MongodbHandlerPool(metaclass=SingletonThreadsafe):
+#class MongodbHandlerPool():
     '''
     '''