traffic-control: Install FqCoDelQueueDisc by default

By default, instead of installing a pfifo-fast qdisc, install an fq-codel qdisc on single-queue devices and an mq qdisc plus as many fq-codel child qdiscs as the number of queues on multi-queue devices (such as wifi).

This change is in line with the current defaults on most Linux distributions. Also, it fixes issue #35 (closed) which highlighted that EDCA differentiation does not work properly due to the different ACs being mixed in the same tc qdisc.

I verified that all tests and examples pass. I restored pfifo-fast for the ns3-tcp-state test, given that such test was designed based on that qdisc.