ovn: Make it possible for CMS to detect when the OVN system is up-to-date.

ovn: Make it possible for CMS to detect when the OVN system is up-to-date.

Until now, there has been no reliable for the CMS (or ovn-nbctl, or
anything else) to detect when changes made to the northbound configuration
have been passed through to the southbound database or to the hypervisors.
This commit adds this feature to the system, by adding sequence numbers
to the northbound and southbound databases and adding code in ovn-nbctl,
ovn-northd, and ovn-controller to keep those sequence numbers up-to-date.

The biggest user-visible change from this commit is new a new option
--wait to ovn-nbctl.  With --wait=sb, ovn-nbctl now waits for ovn-northd
to update the southbound database; with --wait=hv, it waits for the
changes to make their way to Open vSwitch on every hypervisor.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>