[#278,!162] Minor simplification of PacketQueue<> interface

[#278,!162] Minor simplification of PacketQueue<> interface

src/lib/dhcp/packet_queue.h
    PacketQueue<>
        enqueuePacket()
        dequeuePacket() - are now pure virtual,
        and no longer accept a QueueEnd parameter

        shouldDropPacket()
        eatPackets()
        pushPacket()
        popPacket()
        peek() - removed (they are now only in
        PackeQueueRing<> and its derivations

src/lib/dhcp/packet_queue_ring.h
    New fiel which houses PacketQueueRing<> and its derivations