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
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