ability to migrate memfile leases to database leases

The reverse action that #2038 (closed) would take.

Something like: kea-admin lease-import mysql -6 -o leases.csv.

These two kea-admin commands would cover all the cases. If someone wants to migrate from MySQL to PostgreSQL they can go through CSV. That is:

kea-admin lease-dump mysql -6 -o leases.csv
kea-admin lease-import pgsql -6 -o leases.csv

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information