[master] Infrastructure for global host reserverations

- Internal representation of unused subnets is now SunbetID == SUBNET_ID_UNUSED
- Host Reservations with a subnet id value of SUBNET_ID_GLOBAL are global
- kea-dhcp4/6 servers can now parser reservations at the global level

diff --cc src/lib/dhcpsrv/parsers/host_reservation_parser.cc
index 3ec3fd5cca0d2988210ac6dd172e00b5c55ce0cc,2bf08f4dc3725466e564dfbaf5085f9b274275b1..38a8915b41dca44d238fcc0c70805211cf261465
--- 1/src/lib/dhcpsrv/parsers/host_reservation_parser.cc
--- 2/src/lib/dhcpsrv/parsers/host_reservation_parser.cc
+++ b/src/lib/dhcpsrv/parsers/host_reservation_parser.cc
@@@ -1,4 -1,4 +1,4 @@@
--// Copyright (C) 2014-2017 Internet Systems Consortium, Inc. ("ISC")
++// Copyright (C) 2014-2018 Internet Systems Consortium, Inc. ("ISC")
  //
  // This Source Code Form is subject to the terms of the Mozilla Public
  // License, v. 2.0. If a copy of the MPL was not distributed with this