Turn sal-akka-segmented-journal's PerformanceTest into a benchmark

The PerformanceTest we have is useful for diagnosing potential bottlenecks.

Relocate it to benchmark/segjournal-benchmark, making it a self-contained executable jar. The various test options we have should be specifiable with command-line arguments.

The artifact should have a 'benchmarks' profile, which executes the jar with the current set of options (segment/entry/payload sizes).

At the end of the run, the internal histograms should be processed, unlike the current approach where we create the histogram ourselves. Not sure how they are accessible – but that should not be hard to figure out.