queue by list of packets rather than one by one

I have created a branch which greatly simplifies the PacketQueue<> interface to provide implementations with much more latitude. Lower level functions, such as push, peek, and pop were moved down into PacketQueueRing<> (the default implementation base). It also replaces queuing a single packet, to queuing a list of packets. The latter change should reduce thread contention by enabling the receiving thread to read all ready packets and pass them into the queue in one call. Lastly, I split out the PacketQueueRing<> from dhcp/packet_queue.h into its own header, dhcp/packet_queue_ring.h