PID file should not be overwritten if JobScheduler is started twice

Starting Situation

• When starting a JobScheduler instance while another instance is already running, then during the second start the PID file is overwritten. This means the PID file for the running JobScheduler instance is lost.

Desired Behaviour

• Implementation for Unix
  • Write PID file only if the JobScheduler has been started succesfully.
  • Check if a process exists with the PID from the latest PID file and deny starting JobScheduler if such a process exists.
• Implementation for Windows
  • This feature will not be available for Windows due to the fact that a Windows service is used and that no manual JobScheduler start is expected.