Support ramping up the requests over time

[nioncode]

It would be nice if oha would support ramping up the requests over time, so that we can give a rate like --rampup 5 which means to add 5 requests/s, so that it does:

• 0s: 5 requests/s
• 1s: 10 requests/s
• 2s: 15 requests/s
• 3s: 20 requests/s
• ...
• 1000s: 5000 requests/s
• ...

That way one could measure after what time the latencies of a service get worse or how many requests/s it can reasonably handle without blowing up the latencies.