Specify what is logged by the log file
`default_config.txt` talks about logging to a file. I was sure what would be logged, but since there isn't a way to see the hash rate, that it would output a hashrate. Now I'm seeing a log file that looks like:
```
1514864820053;4729
1514864880053;5055
1514864940053;5489
1514865000054;5349
1514865060054;5835
```

I see the first one is a timestamp, but what's the second number? It'd be great if the logging was better documented.