Persistent storage of DHCPv4 and DHCPv6 bindings over EVPN IRB

When configured to function as a DHCP relay agent, can also be configured to preserve the DHCPv4 and DHCPv6 subscriber bindings across reboots. Existing bindings are written to a local file in /var/preserve. After reboot, the binding table is populated with the contents of the file and the router identifies each subscriber that was on the deleted interface, and resumes normal packet processing for subscribers when the interface is restored. To preserve the subscriber binding information, enable the perisistent-storage statement under the [edit system services dhcp-local-server] hierarchy.