[5704] host backends and kea-dhcp4/6 support global HR storage

- Added constants for special SubnetIDs:
    SUBNET_ID_GLOBAL, SUBNET_ID_MAX, SUBNET_ID_UNUSED

- Modified code throughout to use these constants, rather than hard-coded
  values.   Note, MySQL and PostgreSQL host backends convert from NULL to
  UNUSED and back.

- kea-dhcp4/6 servers will now parse a "reservations" element at the global
  level.

src/lib/dhcpsrv/subnet_id.h
    Added constants SubnetID SUBNET_ID_GLOBAL, SUBNET_ID_MAX, SUBNET_ID_UNUSED

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
src/bin/dhcp4/json_config_parser.cc
    kea-dhcp4 parsing now handles reservations as a global element

src/bin/dhcp4/tests/config_parser_unittest.cc
    TEST_F(Dhcp4ParserTest, globalReservations) - new test to
    verify global HR parsing

src/bin/dhcp4/tests/dora_unittest.cc
src/lib/dhcpsrv/cfg_hosts.cc
src/lib/dhcpsrv/host.cc
src/lib/dhcpsrv/host_mgr.cc
src/lib/dhcpsrv/mysql_host_data_source.cc
src/lib/dhcpsrv/parsers/host_reservation_parser.cc
src/lib/dhcpsrv/tests/alloc_engine4_unittest.cc
src/lib/dhcpsrv/tests/alloc_engine_utils.cc
src/lib/dhcpsrv/tests/host_mgr_unittest.cc
src/lib/dhcpsrv/tests/host_reservation_parser_unittest.cc
src/lib/dhcpsrv/tests/host_reservations_list_parser_unittest.cc
src/lib/dhcpsrv/tests/host_unittest.cc
    Replaced SubnetID 0 with SUBNET_ID_UNUSED

src/lib/dhcpsrv/srv_config.cc
    SrvConfig::toElement() - added global reservations output

src/lib/dhcpsrv/tests/cfg_hosts_unittest.cc
    TEST_F(CfgHostsTest, globalSubnetIDs)
    TEST_F(CfgHostsTest, unusedSubnetIDs) - new tests

src/lib/dhcpsrv/tests/host_unittest.cc
    Replaced SubnetID 0 with SUBNET_ID_UNUSED
    TEST_F(HostTest, toText)  - updated to verify global ID output

src/lib/dhcpsrv/tests/mysql_host_data_source_unittest.cc
    TEST_F(MySqlHostDataSourceTest, globalSubnetId4)
    TEST_F(MySqlHostDataSourceTest, globalSubnetId6) - new tests

src/lib/dhcpsrv/tests/srv_config_unittest.cc
    TEST_F(SrvConfigTest, unparseHR) - added global HRs

src/lib/dhcpsrv/testutils/generic_host_data_source_unittest.*
    GenericHostDataSourceTest::testGlobalSubnetId4()
    GenericHostDataSourceTest::testGlobalSubnetId6()

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
src/bin/dhcp6/json_config_parser.cc
    kea-dhcp6 now parses reservations as a global element

src/bin/dhcp6/tests/config_parser_unittest.cc
    TEST_F(Dhcp6ParserTest, globalReservations) - new test

src/bin/dhcp4/dhcp4_parser.h

@@ -1481,7 +1481,7 @@ namespace isc { namespace dhcp {
     enum
     {
       yyeof_ = 0,
-      yylast_ = 970,     ///< Last index in yytable_.
+      yylast_ = 964,     ///< Last index in yytable_.
       yynnts_ = 363,  ///< Number of nonterminal symbols.
       yyfinal_ = 28, ///< Termination state number.
       yyterror_ = 1,