add delay execute parameter

[sn0wfree]

Node(sql)

1. In for loop, may be can add delay arguements to execute sql later! parallel execute all sql at same time after for loop
   such as:

for sql in sql_list:
     Node(sql,delay=True,merged=True) # not real execute sql

Node.delay_tasks.run() # execute all sql which has been mark delay

其中可以加入merge参数，来决定该语句是否可以被合并执行
如果merge=True，可以合并相同的sql语句，只执行一次以减少数据请求次数，但是返回结果样式一致