Cleanup snapshot mr jobs

- Set BytesWritable capacity
- Do not recreate PrioritizedFactBytes
- Rename PrioritizedFactBytes to PriorityTag
- Change outputKey to BytesWritable mutating it for each key
> In the loop im calling writeWith to access the state inside the MutableState

Good point. I guess you could make `Unit` a type, but that's only really useful for testing. /shrug