tcp: add RTAX_FEATURE_TCP_USEC_TS

This new dst feature flag will be used to allow TCP to use usec
based timestamps instead of msec ones.

ip route .... feature tcp_usec_ts

Also document that RTAX_FEATURE_SACK and RTAX_FEATURE_TIMESTAMP
are unused.

RTAX_FEATURE_ALLFRAG is also going away soon.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>