OPENDJ-1910 Fix xxxrate tools stats

* com.forgerock.opendj.ldap.tools.PerformanceRunner
** We start to taking in account garbage collection time when the stats thread starts to run and not when the tool start.

** Refactor the run() method of StatThread to make the code more readable
*** Extract methods to compute stats, prints titles and stats.
*** Extract methods to do ensure that we never divide by zero
** Compute total stats (e.g duration, success, failures) from interval stats and
not from scratch to simplify code
*** Use java.concurrent.TimeUnit where possible

* Other changes are consequences of the previous ones