Separate call rate increase to a dedicated task

For several load tests, I want to have finer-grained statistics coming from SIPp, so I have the `-fd` option set to `1s`.  However, this means that if I want to use the automatic rate increases, I'm stuck with either coarser stats, or a less-useful ramp-up time.

This PR is an attempt to move call rate increases into a separate task.

Note: I have not yet been able to test this because I'm having trouble with the autotools. I'll put those details into a comment.