Ensure no logs are dropped

[fm-117]

As seen in our builds, some logs seem to be dropped while the application, and consequently the LoggingSystem, is exiting.

To fix that:

• in LoggerThread.AddWork: add logging action only when _stop flag is false and throw an exception when it is true
  • This should be fine with our current use as we never had a failure on the Debug.Assert(!_stop);
• in LoggerThread.Stop: at the end check whether the queue is empty or not. If some logging actions are still pending, use current thread to perform them