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
That array has alternating keys and values, so only every other values is expected to be a `String`