OfAgent should be notified of removed logical devices

Anytime a logical device is added/remove in the core, OfAgent updates the device status to ONOS (starting or stopping the OpenFlow connection).

OfAgent keeps track of the logical devices via a polling mechanism: refreshDeviceList

That causes issues if the device reboots faster than the state is updated in ONOS.

The core should notify OfAgent that a device has been removed and considering there is already a gRPC stream open between the two it should easy to add this case: handleChangeEvents