tcp: add CDG congestion control

CAIA Delay-Gradient (CDG) is a TCP congestion control that modifies
the TCP sender in order to [1]:

  o Use the delay gradient as a congestion signal.
  o Back off with an average probability that is independent of the RTT.
  o Coexist with flows that use loss-based congestion control, i.e.,
    flows that are unresponsive to the delay signal.
  o Tolerate packet loss unrelated to congestion. (Disabled by default.)

Its FreeBSD implementation was presented for the ICCRG in July 2012;
slides are available at http://www.ietf.org/proceedings/84/iccrg.html

Running the experiment scenarios in [1] suggests that our implementation
achieves more goodput compared with FreeBSD 10.0 senders, although it also
causes more queueing delay for a given backoff factor.

The loss tolerance heuristic is disabled by default due to safety concerns
for its use in the Internet [2, p. 45-46].

We use a variant of the Hybrid Slow start algorithm in tcp_cubic to reduce
the probability of slow start overshoot.

[1] D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
    delay gradients." In Networking 2011, pages 328-341. Springer, 2011.
[2] K.K. Jonassen. "Implementing CAIA Delay-Gradient in Linux."
    MSc thesis. Department of Informatics, University of Oslo, 2015.

Cc: Eric Dumazet <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Neal Cardwell <[email protected]>
Cc: David Hayes <[email protected]>
Cc: Andreas Petlund <[email protected]>
Cc: Dave Taht <[email protected]>
Cc: Nicolas Kuhn <[email protected]>
Signed-off-by: Kenneth Klette Jonassen <[email protected]>
Acked-by: Yuchung Cheng <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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