Add Step increase feature

A way to give it a Step Period and Step Rate. Example:

`--step-period 30m  --step-rate-increase 1000  --max-requests 20000
`

So you can increase the rate every 30 minutes by 1000 requests. Metrics can settle during the step period and the slow increase of requests can let you know the tipping point of you application. The max requests can have no ceiling by default.