Implement OFPT_TABLE_STATUS Message.

Implement OFPT_TABLE_STATUS Message.

On change in a table state, the controller needs to be informed with
the OFPT_TABLE_STATUS message. The message is sent with reason
OFPTR_VACANCY_DOWN or OFPTR_VACANCY_UP in case of change in remaining
space eventually crossing any one of the threshold.

Signed-off-by: Saloni Jain <saloni.jain@tcs.com>
Co-authored-by: Rishi Bamba <rishi.bamba@tcs.com>
Signed-off-by: Rishi Bamba <rishi.bamba@tcs.com>
[blp@ovn.org added vacancy event initialization and tests
 and updated NEWS]
Signed-off-by: Ben Pfaff <blp@ovn.org>