Checking in experimental chunked-download server code
Added command-line option to client to request a chunk (Currently does nothing)
OPT_CHUNKED_DOWNLOAD sessions no longer have Seq or Ack values on the server side
Moved all the code that acts on 'Packet's (the class, not the concept) into the session code and out of the driver code
No visible changes, but packets' 'options' are now integrated throughout the code, getting ready to add the new chunked download code
Added more features that will let us use chunked encoding soon
Cleaned up some session-management code that was getting outta hand