Support Running Experiments in Parallel

[pfleck]

Before #2258, the Optimizers in an Experiment could be run in parallel by running the experiment and then starting some Optimizers manually. For each manually started Optimizer, a "Worker Thread" is spawned implicitly that (after the manually started Optimizer finished) assists executing the remaining Optimizers of the Experiment.

To support synchronous calls of Optimizer.Start (implemented in #2258), the behavior was changed and a "Worker Thread" does not run other Optimizers than the Optimizer it was spawned for.

To allow the user to run an experiment in parallel, we should introduce a more intuitive way of paralleling the experiment, similar as the "Workers" within a CrossValidation.