Create y_think_time_for_rampup() and y_think_time_for_rampup_ext().

Create y_think_time_for_rampup() and y_think_time_for_rampup_ext(). Remove the previous calculating versions.

Instead of using a fixed user-provided response time we can also just measure time between calls and use that instead. So we remove the response tiem argument, do the think time ourselves, and do a time measurement afterwards.

This has the nice side effect that changes in script duration (wasted time) will have less effect on the load.