tornado: 一次request从开始到结束

一个简单的调用顺序（只包含了重要的方法）：

On IOLoop.READ triggered:

1. TCPServer._handle_connection

2. HTTPServer.handle_stream

3. HTTPConnection.__init__

4. HTTPConnection._on_headers

5. HTTPConeection._on_request_body

6. Application.__call__

7. RequestHandler._execute

8. RequestHandler(or anyother child of it)'s get/post/delete/head/... method

9. RequestHandler.finish