Improve percentiles computation with HDR histogram

This commit also introduces new fields in the Metrics.Latencies struct.
* Min
* P999
@codahale: Even after the last commit, the numbers are still sketchy with `sigfigs=5`
```
Latencies     [mean, min, 50, 95, 99, 999, max]  -2562047h47m16.854775808s, 179.401us, 0, 0, 0, 0, 9.405753ms
```