cc_cubic: Use units of micro seconds (usecs) instead of ticks in rtt.

This improves TCP friendly cwnd in cases of low latency high drop rate
networks. Tests show +42% and +37% better performance in 1Gpbs and 10Gbps
cases.

Reported by: Bhaskar Pardeshi from VMware.
Reviewed By: rscheff, tuexen
Approved by: rscheff (mentor), tuexen (mentor)