Update logger to use file handles

[roop]

Revamped Logger:

• Logging operations directly write to a file on disk, so no log entries are discarded because of "too many lines in memory"
• File writes / flushes are queued to avoid crashes / races

[roop]

@jeroenleenarts The app version is logged pretty much like before -- at the start of the log. Now, LoggingService uses Logger, and when it creates the Logger instance, it logs the app version. So there's not much of a change in behaviour in this aspect.