Have a DSL for pauses, close #1179

Pause strategy is now to be configured on set up, not on per-request basis:
Here are the alternatives:

```scala

.pauses(Constant) // default
.pauses(Exponential)
.pauses(Custom()) // bypass value set in pause
.pauses(UniformPercentage(percentageToComputeHalfWith)
.pauses(UniformDuration(halfWithDuration)
```

WDYT?