Write section about backups

[JonasAlfredsson]

We should probably add a section explaining how to back up the memory file, since this appears to be non-trivial if the service is running.

[JonasAlfredsson]

This hunt led me to this thread, and specifically this comment: opnsense/core#7204 (comment)

Where the most insightful explanation comes from here: https://gitlab.isc.org/isc-projects/kea/-/wikis/designs/Lease-File-Cleanup-design

[JonasAlfredsson]

From my reading it appears like the dhcp4.csv.2 file should contain the cleaned up leases state since the last time kea-lfc ran, and this process is automatically triggered by Kea once every hour (default lfc-interval).

The main dhcp4.csv is just the journal of the changes that has happened since the last time it was run, so it is no point in performing a backup just of that one.

Then there are the ".1", ".output" and ".completed" files created by the LFC process, but they are just temporary and should not be present after a successful cleanup.

[JonasAlfredsson]

Some more info regarding file endings used: https://github.com/isc-projects/kea/blob/331e325d1fbf554b9a0088f900f7e4f16d01d678/src/lib/dhcpsrv/memfile_lease_mgr.h#L967-L974

[JonasAlfredsson]

The command run by the Kea process:

executing Lease File Cleanup using: /usr/local/sbin/kea-lfc -4 -x /kea/leases/dhcp4.csv.2 -i /kea/leases/dhcp4.csv.1 -o /kea/leases/dhcp4.csv.output -f /kea/leases/dhcp4.csv.completed -p /kea/leases/dhcp4.csv.pid -c ignored-path