Use lease-commands API to add leases directly to running Kea
If we extend this to allow it to use leaseX-add commands, it could be made to add leases directly to a running Kea. The add commands will try to match to a subnet-id if it is not supplied in the command arguments. This would allow us to load leases into a running Kea regardless of where Kea is storing them (memfile, MySQL, PostgreSQL) and eliminates the need to rely on Memfile sanity checking to backfill subnet IDs. You could also use this to address adding leases to HA peers by sending the commands to all the peers.