Unify a format of log messages

When a MySQL server was available during a startup but the connect has lost after that, it's possible to see the following messages in the log:
```
2019/12/05 22:26:21 main.go:79: Running the server on port 8080
[mysql] 2019/12/05 22:29:08 packets.go:36: unexpected EOF
2019/12/05 22:29:08 main.go:71: Scan() has failed: invalid connection
```
