[packet-queue-queue-by-list] PacketQueue now queues lists of packets

[packet-queue-queue-by-list] 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