kea-dhcp4 will parse and use "queue-control" to configure ring
    buffer size:

    "Dhcp4":
    {
        "queue-control": {
            # max number of packets the ring will hold
            "capacity" : 100
        },
    :

    This is an interrim commit for testing purposes.
    Expect a fair amount of refactoring in subsequent commits.

New files:
	src/lib/dhcp/queue_control.cc
	src/lib/dhcp/queue_control.h
	src/lib/dhcp/tests/queue_control_unittest.cc
	src/lib/dhcpsrv/parsers/queue_control_parser.cc
	src/lib/dhcpsrv/parsers/queue_control_parser.h

src/bin/dhcp4/ctrl_dhcp4_srv.cc
    ControlledDhcpv4Srv::processConfig() -
        added logic to set packet queue controller

src/bin/dhcp4/dhcp4_lexer.ll b/src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
src/bin/dhcp4/parser_context.*
    Added queue-control parsing

src/bin/dhcp4/json_config_parser.cc
    configureDhcp4Server() - recognize and parse "queue-control"

src/bin/dhcp4/tests/config_parser_unittest.cc
    TEST_F(Dhcp4ParserTest, queueControl)
    TEST_F(Dhcp4ParserTest, queueControlInvalid)
    - new tests

src/lib/dhcp/iface_mgr.*
    IfaceMgr::getPacketQueueControl4()
    IfaceMgr::setPacketQueueControl4()
    IfaceMgr::getPacketQueueControl6()
    IfaceMgr::setPacketQueueControl6()

src/lib/dhcp/packet_queue.*
    Use QueueControl class

src/lib/dhcpsrv/srv_config.*
    Added QueueControl member, getter/setter
    SrvConfig::toElement() - now emits queue-control if not null