使用将请求写入文件的方式启动服务，写入的文件tail -f时无法监控。

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#### Which platform are you running AnyProxy

platfrom:ubuntu 16.04
#### The version of the AnyProxy

version:3.10.3
#### Your expected behavior of AnyProxy

anyproxy --intercept --file /var/log/requests.txt --port 9999
期望在抓包写入文件的同时，tail -f可以实时监控到。
#### The actual behavior of AnyProxy

tail -f /var/log/requests.txt
现在在写入文件的过程中，tail -f 无法监控到文件内容的变化。
#### The log of the error

none

Reproduce the issue on Ubuntu 16.04 with AnyProxy 3.10.3 using `anyproxy --intercept --file /var/log/requests.txt --port 9999`, then monitor the file with `tail -f`. Trace the request-file writing entry point and verify that requests become visible while the service is running; done means the file updates are observable in real time.

Written by the indexing model from the issue text.