ISC DHCP release on roam flag

          release-on-roam flag;

          When enabled and the dhcpd server detects that a  DHCPv6  client
          (IAID+DUID)  has  roamed  to  a new network, it will release the
          pre-existing leases on the old network and emit a log  statement
          similiar to the following:

                "Client:  <id>  roamed  to  new  network, releasing lease:
          <address>"

          The server will carry out all of the same steps that would  nor-
          mally  occur  when  a  client explicitly releases a lease.  When
          release-on-roam is disabled (the default) the server makes  such
          leases unavailable until they expire or the server is restarted.
          Clients that need leases in  multiple  networks  must  supply  a
          unique IAID in each IA.  This parameter may only be specified at
          the global level.

Make sense for a moving client which for some reasons does not release its address at the previous location. Can be implemented in Kea if there is an easy and fast way to get leases by IAID+DUID only.