+pro,ben #15180 add akka-bench-jmh, enable running jmh benchmarks form sbt

Working integration with JMH.

Details in commits - it's now runnable by just going to the `akka-bench-jmh` (better name ideas?) project and running `run [any jmh options]`. Example: `run -i 3 -wi 3 -f 1 .*actor.*`, for understanding the options simply pass in `-h`.

// btw "why not a loop but JMH": http://hg.openjdk.java.net/code-tools/jmh/file/f2e982b7c51b/jmh-samples/src/main/java/org/openjdk/jmh/samples/JMHSample_11_Loops.java

Eventually will resolve #15180