Implement Mapping class sharing multiple queues for best effort

[xtor]

This class should allow to take advantage of the implicit round-robin in taprio mode 0x2 when multiple queues are associated to the same traffic class.

In principle, this should allow to place streams or groups of streams anywhere in the cycle with i226.

One illustration of such schedule making use of this mapping would be:

sudo tc qdisc replace dev ${INTERFACE} parent root taprio \
              num_tc 3 \
              map 0 1 2 0 0 0 0 0 \
              queues 2@2 1@0 1@1 \
              base-time 0 \
              sched-entry S 08 200000 \
              sched-entry S 01 200000 \
              sched-entry S 02 200000 \
              sched-entry S 04 200000 \
              flags 0x2