Threading support for SOM benchmark harness

These commits add support to execute benchmarks using threads.

Note, benchmarks need to be inherently thread-safe, and for instance the use of a stateful random number generator will lead to bugs.

With the new support, a benchmark can be execute like this:

```./som.sh -cp Smalltalk:Examples/Benchmarks/DoesNotUnderstand Examples/Benchmarks/BenchmarkHarness.som Dispatch 10 0 5 2```

Which executes the Dispatch benchmark on 2 threads, each thread is going to do 10 iterations of the benchmark, with 0 warmup iterations, and 5 iterations in the inner benchmark loop.

TODO: the reporting of the over all result is currently broken, but shouldn't be relevant for our purposes currently.