Add locks on getLoggerInstance

[birschick-bq]

If multiple threads try to get the logger instance, 2 logger instances might be created at the same time. This may or may not be an actual issue. Note that locks would impair the run time.

Example:

  // pseudo-code
  getLoggerInstance() {
  // get lock
    if (!_logger)
      _logger =  new Logger();
    // unlock
    return _logger;
  }

AC:

• add lock protection to getLoggerInstance