Replace Google Caliper with Oracle JMH

Dear Coda,

There is a recent tool from Oracle JVM engineers [Oracle JMH](http://openjdk.java.net/projects/code-tools/jmh) , which is used for JDK benchmarking and verification. It has the advantage over Caliper by options for organizing multi-threaded benchmarking.

I've applied it on the current `update()` tests as [metrics-core-benchmarking](https://github.com/ceetav/metrics-core-benchmarking) with light syntax adaptation.  The sample benchmarking output is available [here](https://gist.github.com/ceetav/55de5d8e9f7cd785be2f) and it looks more informative especially for contention examination.

Do you think it make sense for the pull request?

Regards,
ceetav