...so that all the subclasses have such attributes. This
allows queue discs to have attributes specifying the mode and
size of their queue(s) and to create queues using their
own attributes.
This commit is heavily inspired by Natale's queue rework patch:
https://codereview.appspot.com/270540044/
A QueueItem base class is introduced to represent the items stored
in a Queue. The base class only contains a Ptr<Packet>. Derived classes
can store additional information. DropTailQueue, RedQueue and CodelQueue,
along with their examples and testsuits, have been adapted. Objects using
such queues have been adapted too.
