Enhance benchmarking

:Release Notes:
Better reproducibility of benchmarking tests

:Detailed Notes:
- process speedup by avoiding extra cycle if warmup already hit the
  measure_time
- tolerance to system load by using CPU usage time if available
- tolerance to spikes by using median instead of average

:Testing Performed:
gtest

:QA Notes:

:Issues Addressed:
[BHV-123] pbnjson: Improve performance of json parsing with YAJL

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Open-webOS-DCO-1.0-Signed-off-by: Andrii Zora <andrii.zora@lge.com>

Change-Id: I661407ce08ef807361cb481578674f845c18570e
Reviewed-on: https://g2g.palm.com/4540
Reviewed-by: DCO Verification
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>