Dealgood: add adaptive throughput capability

[iand]

Currently experiments require the rate of requests and concurrency (requests in-flight) to be specified as part of the experiment. This takes some manual experimentation to get right. Dealgood could adapt the rate and/or concurrency dynamically using a threshold on either the drop rate or the error rate from the target to find the best continuous throughput for the experiment.

[thattommyhall]

This is an excellent idea, the arrived at throughput/concurrency could be deemed the result of the experiment even