Better simulate real load

With RTMP, stream-sender doesn't have control over segmentation process, so each time it streams into B, it gives exactly same load.
With HTTP, stream-tester doing segmentation itself, so it gives ability to segment stream differently, even randomising segment's lengths during one stream.

So now we can implement Josh simulation algorithm from [here](https://github.com/livepeer/go-livepeer/issues/1112) but using real streams.

* New streams arrive as part of a Poisson process
* Stream lengths are normally distributed (median, say, ~"45 mins" with a slightly fatter tail at each end)
* Each stream has a random set of transcoding renditions : min 2, max 5, selected from videoprofile.go (uniformly?)
* Stream segment lengths are selected from: [2s, 4s, 10s] with weights of [0.8, 0.15, 0.5] respectively. (Could also try totally randomizing this between 2-10s.)
* Segment lengths within a stream are exponentially distributed.
* Segments generally "arrive" one after the other with a small amount of jitter.

Also we should create mode in which load will be gradually increased util it falls below specified threshold.