neutron-server high memory usage due to OVN mac_binding explosion at scale

Issue

• In a large-scale environment, growth of OVN southbound database MAC_Binding table can cause excessive Neutron RSS memory consumption and trigger out-of-memory killer (OOM killer).
• For example, the neutron server used 73.5G of RSS memory in a deployment with 4k VMs, 1.45K networks, 2.4K floating IPs, 800 routers, 820 security groups and 3.3K security groups rules. The 800 routers were connected to a /16 external network to serve floating IPs from this network for the VMs. The OVN southbound database had 1.15 million MAC_binding rows.
• Memory consumption did not come down after cleanup of all routers, VMs, and other neutron resources.