[#1307] kea-dhcp6 supports parked-packet-limit

[#1307] kea-dhcp6 supports parked-packet-limit

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
src/bin/dhcp6/json_config_parser.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
doc/examples/kea6/all-keys.json
    Added parked-packet-limit

src/bin/dhcp6/dhcp6_messages.*
    DHCP6_HOOK_LEASES6_PARKING_LOT_FULL

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::processDhcp6Query() - drops the packet if the parking
    lot is full

src/bin/dhcp6/tests/get_config_unittest.cc
    updated