TCP Cubic: improve reaction to (and rollback from) RTO

[chengc_netapp.com]

Summary

1. fix compliancy issue of CUBIC RTO handling according to RFC8312 section 4.7
2. add CUBIC CC_RTO_ERR handling

This is the second patch of two consecutive patches on CUBIC to improve
reaction to (and rollback from) RTO.

Test Plan

Performed multiple unit tests and inspected the evolution of the cubic-internal
state across packet RTO/RTO_ERR scenarios, by using Emulab.net.

[rscheff]

discussed this internally, but want to have external review before commit.