Improve percentiles computation with HDR histogram

This commit also introduces new fields in the Metrics.Latencies struct.
* Min
* P999
The trivial way to simulate the load test in question is to ^Z the server being tested for 20 seconds, while your load generator is trying to generate 200K reqs/sec. If at the end of the test the number of issued request is equal to actual_run_time * 200K, then you are not dropping things. If it's not, you have s problem.

You can also use this test to sanity check your reported latencies. You can hand- compute some percentiles from the known behavior. E.g. If the total run time is 40 seconds and the pause is 20 secinds, your 99%'ile should show 19.6 seconds, your 95%'ile should show 18 seconds, etc. If that's not what theyvshow, they are wrong.