tcp: (fixes #805) Update NextSeg() condition for SACK as per RFC
Added a member variable m_sackSeen
to ns3::TcpTxBuffer
and updated condition for NextSeg() to align with RFC 6675 as per section (1.b).
Added a member variable m_sackSeen
to ns3::TcpTxBuffer
and updated condition for NextSeg() to align with RFC 6675 as per section (1.b).