Add command to clean up expired sessions

[jphalip]

Sessions that don't explicitly get cleaned (e.g. if the Yarn Resource Manager fails to call the "cancel" method), then some expired sessions will remain in the database. Those sessions could be cleaned up by running a clean-up command on a regular basis with a cron job. Something like what Django has:
https://docs.djangoproject.com/en/dev/topics/http/sessions/#clearing-the-session-store
https://docs.djangoproject.com/en/dev/ref/django-admin/#django-admin-clearsessions

[jphalip]

The "creation_time" column should also be indexed to improve performance for that command.