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