Removed debug output

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Added support for parallel execution of benchmarks

This closes #40.

The configuration settings are chosen conservatively, so that benchmarks will be executed sequentally.

The configuration has two new elements:
- `runs: parallel_interference_factor:` defines a factor that is used to determine the number of worker threads based on the available cores. Increasing the factor will reduce interference.

- `vm:execute_exclusively:` a VM can indicate that all the benchmarks relying on it should be executed without any other parallel running benchmarks. Since this is set to true per default, execution will only be parallel if specifically requested.

Note, this change comes without tests.

Signed-off-by: Stefan Marr <git@stefan-marr.de>