ovn-controller: Monitor only necessary southbound rows.

ovn-controller: Monitor only necessary southbound rows.

Until now, ovn-controller has replicated all of the southbound database
(through the IDL).  This is inefficient, especially in a large OVN setup
where many logical networks are not present on an individual hypervisor.
This commit improves on the situation somewhat, by making ovn-controller
replicate (almost) only the port bindings, logical flows, and multicast
groups that are actually relevant to the particular hypervisor on which
ovn-controller is running.  This is easily possible by replicating the
patch ports from the Port_Binding table and using these relationships to
determine connections between datapaths.

This patch is strongly influenced by earlier work from the CCed developers.
I am grateful for their assistance.

CC: Darrell Ball <dlu998@gmail.com>
CC: Liran Schour <LIRANS@il.ibm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Liran Schour <lirans@il.ibm.com>
Acked-by: Mickey Spiegel <mickeys.dev@gmail.com>