[inetstack] Add Optional TCP Feature - SACK (Selective Acknowledgement)

[BrianZill]

TCP has an optional feature called Selective Acknowledgement (SACK) which allows a receiver to acknowledge chunks of data, rather than just a single cumulative value. In certain situations, this can save a sender from a large number of needless retransmissions.

This feature is especially useful on somewhat lossy long (high RTT) fat (high bandwidth) networks. Since data centers tend to have low loss, short fat networks, this may not be a high priority for us if we are primarily targeting data center deployments.

See RFC 2018 and RFC 2883 for complete description of SACK. Also see RFC 6675 for a SACK-based loss recovery algorithm.