ofproto: New feature to notify controllers of flow table changes.

OpenFlow switching monitoring and controller coordination can be made more
efficient if the switch can notify a controller of flow table changes as
they occur, rather than periodically polling for changes.  This commit
implements such a feature.

Feature #6633.
CC: Natasha Gude <natasha@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>