This patch adds a NetDeviceQueue class to store information about a single
transmission queue of a network device. This class is meant to store the
state of a transmission queue (i.e., whether the queue is stopped or not)
and some data used by techniques such as Byte Queue Limits. Also, multi-queue
aware queue discs can aggregate a child queue disc to an object of this class.
These features (excluding BQL) are added in subsequent commits.
The NetDevice class maintains a vector of NetDeviceQueue pointers, one for
each transmission queue. A NetDevice constructor is added which creates a
single transmission queue by default for every device. The number of transmission
queues can be modified (by child classes) by calling NetDevice::SetTxQueuesN.
Two public methods, GetTxQueue and GetTxQueuesN, are also added to the NetDevice class
to return the i-th NetDeviceQueue and the number of transmission queues, respectively.
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