TCP Cubic: Have Fast Convergence Heuristic work for ECN, and align concave region

The Cubic concave region was not aligned nicely for the very first exit from
slow start, where a 50% cwnd reduction is done instead of the normal 30%.

This addresses an issue, where a short line-rate burst could result from that
sudden jump of cwnd.

In addition, the Fast Convergence Heuristic has been expanded to work also
with ECN induced congestion response.

Submitted by: chengc_netapp.com
Reported by: chengc_netapp.com
Reviewed by: tuexen (mentor), rgrimes (mentor)
Approved by: tuexen (mentor), rgrimes (mentor)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D25976