[#42,!103] Initial receiver thread and packet queuing

[#42,!103] Initial receiver thread and packet queuing

New files:
    src/lib/dhcp -
    packet_queue.h - defines packet queuing template classes
    socket_info.h - contains existing class extracted iface_mgr.h
    tests/packet_queue4_unittest.cc
    tests/packet_queue6_unittest.cc

src/lib/dhcp/iface_mgr.*
IfaceMgr:: - new functions
    - receiveDHCP<4/6>Packets() - thread worker function which
    monitors interface sockets, enqueues packets as they are read

    - receiveDHCP<4/6>Packet() - reads a single packet from a socket

    - startDHCPReceiver(const uint16_t family) - runs
    receiveDHCP<4/6?appropriate worker function in a thread

    - stopReceiver() - stops the receiver thread

    - setPacketQueue<4/6> - replaces the default packet queue instance

    receiveDHCP<4/6>() - modified to monitor receiver watch
    socekts rather than interface sockets.  Dequeue packets
    from packet queue.

src/lib/dhcp/tests/iface_mgr_unittest.cc
    TEST_F(IfaceMgrTest, packetQueue4)
    TEST_F(IfaceMgrTest, packetQueue6)

src/lib/dhcpsrv/cfg_iface.cc
    CfgIface::openSockets() - starts DHCP receiver
    CfgIface::closeSockets() - stops DHCP receiver