Modify TCP receive buffer size auto scaling to use the smoothed RTT

Modify TCP receive buffer size auto scaling to use the smoothed RTT
(SRTT) instead of the timestamp option.  Since the timestamp option is
disabled on some OSs (eg. Windows) or dropped by some
firewalls/routers, in such a case the window size had been fixed at
16KB, this limits throughput at very low on high latency networks.
Also replace "tcp_now" from 2HZ tick counter to binuptime in
milliseconds to calculate the SRTT better.

tested by krw matthieu jmatthew dlg djm stu stsp
ok claudio