There are congestion control algorithms will that pull in srtt, and this can cause issues with rack.

When using rack, cubic and htcp will grab the srtt, but they think it is in ticks. For rack
it is in micro-seconds (which we should probably move all stacks to actually). This causes
issues so instead lets make a new interface so that any CC module can pull the srtt in
whatever granularity they want.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D40146