Cleanup snapshot mr jobs

- Set BytesWritable capacity
- Do not recreate PrioritizedFactBytes
- Rename PrioritizedFactBytes to PriorityTag
- Change outputKey to BytesWritable mutating it for each key
I really don't mind on granularity in this case because they are only the interfaces, just want it to be clear. So by _struct_, by _concept_, or by _module_ would all be fine for me, but at the moment it is kind of all over the place.