Include more detailed latency output in CSV file

[cpeel]

Currently the elbencho CSV results file includes min, avg, and max latency numbers for IOs and entries. Unfortunately for some tests the max latency really skews the average latency.

elbencho offers more detailed latency results to STDOUT with --latpercent and --lathisto but these numbers aren't stored in the CSV.

It would be ideal if the more detailed latency metrics were saved in the CSV results file in some way.