- Add flow and port stats polling intervals in config.ini

- Add flow and port stats polling intervals in config.ini
- Add DESC stats polling interval in CONFIG.ini and thru OSGI cmd
- When the switch port is changed to admin down or the link is changed to down state, the switch sends port_status msg with OFPPR_MODIFY type. The existing logic is to map OFPPR_MODIFY to UpdateType.CHANGED. But we should exam the Config state and Link state fields within the port_status packet. If they are "down", we should fire an UpdateType.REMOVED event instead and remove both edges associated with the port.

Signed-off-by: Jason Ye <yisye@cisco.com>