[Cake] Running Cake at long RTTs

[Toke Høiland-Jørgensen]

So Dave and I did a bit of testing of Cake at a 1-second base RTT. That
showed that, compared with a straight FIFO queue (with a sufficiently
large buffer), throughput was suffering quite a bit under Cake,
especially at large bandwidths. We did two changes to fix this:

- Turn the hard packet queue size into a lower bound rather than an
  upper bound.

- Scale the target to be 1/16th of the interval.

The first change allows Cake to actually reach the target throughput,
but it still takes a long while to get there. With the second change, we
actually get the desired behaviour. The attached plot shows the
difference, with the solid line being before the change and the dashed
line being after the change.

Patch attached.

-Toke

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