Improve percentiles computation with HDR histogram

This commit also introduces new fields in the Metrics.Latencies struct.
* Min
* P999
In the specific case where the load gen is truly constant throughput (i.e. will speed up to catch up if it falls behind), you can avoid using correction/compensation in the histogram by computing the latency correctly:
Latency = requestEndTime - (systemStartTime + requestNumber / throughput);

This strongly depends on the loader actually working to stay on pace. It wouldn't be valid otherwise.