RFR: 8232594: Make the output of the JFR command duration more user friendly

[Chihiro Ito]

Hi,

I tried to implement this to output an accurate time duration.
Could you review this, please?

Is a rounded duration better than an exact duration?

JBS https://bugs.openjdk.java.net/browse/JDK-8232594
webrev http://cr.openjdk.java.net/~cito/JDK-8232594/webrev.00/

Regards,
Chihiro