Feature: Linear cps increase

This PR allows scaling up the CPS rate over time, linearly.

However, I'm not quite happy with it because the increment interval is hard-coded at 1 second.  This means going from 5CPS to 35CPS can only happen in 30 seconds, which isn't terribly useful to watch the impact of growing CPS. The problem is SIPp, which re-uses the stats interval (`-fd`) setting to determine the CPS increase interval.  The correct fix for this would be to create a separate interval argument to SIPp and use that.  TBD.