OPENDJ-1549 (CR-4695) Refactor the way to calculate xxxrate tools eTimes

* PerformanceRunner.java
Implements a new way to compute eTimes by using "buckets" representing different eTimes ranges and maintain frequency counts.
The warmup duration does now correctly clear eTimes.

git-svn-id: https://svn.forgerock.org/opendj/trunk@11080 41b1ffd8-f28e-4786-ab96-9950f0a78031