Dynamic (re)assignment of processors to slow steps during import

- Made property encoding parallelizable in the batch importer
- Allows for multiple execution monitors
- The "average" stat is now a moving average which responds more quickly to changes
- Added a processor execution monitor that dynamically, throughout the import, (re)assigns processors to steps as it sees fit
Somehow I found it hard, dunno why exactly. Tests should not have anything to do with sleeping in general. The sleep here is for awaiting threads to come into a certain state, which is not as harmful as sleeps in other occasions. But could probably be solved in a different way.