ovn: Change strategy for tunnel keys.

ovn: Change strategy for tunnel keys.

Until now, OVN has used "flat" tunnel keys, in which the STT tunnel key or
Geneve VNI contains a logical port number.  Logical port numbers are unique
within an OVN deployment.

Flat tunnel keys have the advantage of simplicity.  However, for packets
that are destined to logical ports on multiple hypervisors, they require
sending one packet per destination logical port rather than one packet per
hypervisor.  They also make it hard to integrate with VXLAN-based hardware
switches, which use VNIs to designate logical networks instead of logical
ports.

This commit switches OVN to a different scheme.  In this scheme, in Geneve
the VNI designates a logical network and a Geneve option specifies the
logical input and output ports, which are now scoped within the logical
network rather than globally unique.  In STT, all three identifiers are
encoded in the tunnel key.

To allow for the reduced amount of traffic for packets destined to logical
ports on multiple hypervisors, this commit also introduces the concept
of a logical multicast group.  The membership of these groups can be set
using a new Multicast_Group table in the southbound database (and
ovn-northd does use it starting in this commit).

With multicast groups alone, it would be difficult to implement ACLs,
because an ACL might disallow only some of the packets being sent to
a remote hypervisor.  Thus, this commit also splits the OVN logical
pipeline into two pipelines: the "ingress" pipeline, which makes the
decision about the logical destination of a packet as a set of logical
ports or multicast groups, and the "egress" pipeline, which runs on the
destination hypervisor with the multicast group destination exploded into
individual ports and makes a final decision on whether to deliver the
packet.  The "egress" pipeline can efficiently apply ACLs.

Until now, the OVN logical and physical pipeline implementation was not
adequately documented.  This commit adds extensive documentation to
the OVN manpages to cover these issues.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>