tcp: (fixes #407) Align TcpCubic::GetSsThresh() with Linux

Replaced segInFlight with segCwnd in Tcp-Cubic.cc and removed its declaration because it was now an unused variable.

Updated the validation checks. To decide upon the validation criteria, I opened up the trace files generated in both cases and spotted the exact time of the drops of the cwnd. I added minute padding of time in the same order of magnitude as was present earlier and arrived at the lower bound for cwnd in that interval this way.

For instance, if the congestion window started dropping at 14.1948th second, I considered the time interval to be (14.0,14.197) and the minimum cwnd value for the validation check (224) was arrived upon based on that.

The instant at which the congestion window drops was very slightly different for both the cases (cubic-50ms-ecn and cubic-50ms-no-ecn) so the common intersection of intervals arrived at for both cases was used to make the validation test.