[#260,!20] Packet queueing is now optional

[#260,!20] Packet queueing is now optional

src/bin/dhcp<4/6>/ctrl_dhcp<4/6>_srv.cc
    ControlledDhcpv<4/6>Srv::processConfig() - now calls
        IfaceMgr::configureDHCPPacketQueue

src/bin/dhcp<4/6>/dhcp<4/6>_parser.yy
    dhpc-queue-control parsing updated to enforce
    enable-queue/queue-type rules

src/bin/dhcp<4/6>/tests/config_parser_unittest.cc
    TEST_F(Dhcp<4/6>ParserTest, dhcpQueueControl)
    TEST_F(Dhcp<4/6>ParserTest, dhcpQueueControlInvalid)

src/lib/dhcp/iface_mgr.*
    IfaceMgr
    - closeSockets()  - now calls stopDHCPReceiver

    - openSockets<4/6>() -  now calls startDHCPReceiver

    - receive<4/6>Indirect() - new function which monitors receiver
    thread watch sockets, reads DHCP packets from queue

    - receive<4/6>Direct() - new function which monitors and reads DHCP
    packets from interface sockets directly

    - receive<4/6>() - rewritten to call receive<4/6>Indirect
    if receiver thread is running, otherwise it calls receive<4/6>Direct

    - configureDHCPPacketQueue() - new function which either enables queuing
    by creating a new packet queue, or disables it by destroying the
    existing queue

src/lib/dhcp/packet_queue_mgr.h
    PacketQueue::destroyPacketQueue() - new function

src/lib/dhcp/packet_queue_mgr<4/6>.cc
    PacketQueueMgr<4/6>::PacketQueueMgr<4/6>() - no longer creates a
    default packet queue

src/lib/dhcpsrv/cfg_iface.cc
    CfgIface::closeSockets() - removed call to stopDHCPReceiver
    CfgIface::openSockets() - removed call to startDHCPReceiver

src/lib/dhcpsrv/parsers/dhcp_queue_control_parser.*
    DHCPQueueControlParser
    - removed unused family_ member
    - parse() - added support for enable-queue

src/lib/dhcpsrv/tests/dhcp_queue_control_parser_unittest.cc
    - new file