[#307,!150] Packetqueue-interface-clean-up

 PacketQueue now queues lists of packets

    src/lib/dhcp/Makefile.am
        Added packt_queue_ring.h

    src/lib/dhcp/iface_mgr.*
        IfaceMgr::receiveDHCP4Packets()
        IfaceMgr::receiveDHCP6Packets() - modified to queue a list of packets

        IfaceMgr::receiveDHCP6Packet() - modified to return packet rather than
        queue it

    src/lib/dhcp/packet_queue.h
        PacketQueue<>
            - replaced enqueuePacket() with enqueuePackets() and is
            now pure virtual
            - dequeuePacket() is now pure virtual
            - Removed pop, push, peek methods

        Moved All PackeQueueRing<> code into new header

    src/lib/dhcp/packet_queue_ring.h
        New header which contains PacketQueueRing<> and derivations