Log file should be flashed on linebreaks

Look at the last truncated line that stayed this way for a while:

```
# tail -3 /var/log/zeronet.log
[13:31:33] FileServer FileRequest: Conn#2223 zvnzp4c26x6hkizm.onion [v2] pex 1Apr5ba6u9Nz6eFASmFrefGvyBKkM76QgE None
[13:31:39] FileServer Conn#3427 181.28.181.23 [v2] > Closing connection: MessageLoop ended, waiting_requests: 0, sites: 1, buff: 0...
[13:[root@yuri /home/yuri]#
```

### Step 1: Please describe your environment

* ZeroNet version: 0.6.0
* Operating system: FreeBSD 11.1

The report provides only a truncated /var/log/zeronet.log example and ZeroNet 0.6.0 on FreeBSD 11.1; no source file or test is identified. First reproduce the linebreak behavior and locate the component that writes this log, then verify that completed lines are flushed and the prompt no longer appears on the same line.

索引モデルが issue の本文から書いたものです。