[inetstack] Add Optional TCP Feature - Timestamps

[BrianZill]

TCP has an optional "Timestamps" feature which provides a mechanism for taking considerably more accurate time measurements (used to determine the round-trip time, or RTT). Accurate and timely RTT measurements are important for determining the current retransmit timeout (RTO) value.

Support for the Timestamps feature is also a prerequisite for supporting the "Protection Against Wrapped Sequences" (PAWS) mechanism, which protects against errors due to sequence number wrap-around on high-speed connections.

See RFC 7323 for more information.